Fix validation error type `GL03` and check in CI
#### Problem description

In order to have a continuous check by the CI on validation error `GL03` (Double line break found; please use only one blank line to separate sections or paragraphs, and do not leave blank lines at the end of docstrings), fixing them in the code base enables the addition to the CI for automated testing in the future.

`GL03` has 99 occurrences (see [overview of the errors](https://gist.github.com/198cfa552c727d71bd7360fda66fdbb4)), including:


| method | file | file_line | github_lin
|  --- | --- | --- | --- 
pandas.Timedelta | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.Timedelta.max | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.Timedelta.min | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.SparseArray | pandas/core/arrays/sparse.py | 505.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/sparse.py#L505
pandas.DataFrame.agg | pandas/core/frame.py | 6291.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6291
pandas.DataFrame.aggregate | pandas/core/frame.py | 6291.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6291
pandas.DataFrame.compound | pandas/core/generic.py | 10038.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10038
pandas.DataFrame.cummax | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.DataFrame.cummin | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.DataFrame.cumprod | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.DataFrame.cumsum | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.DataFrame.kurt | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.DataFrame.kurtosis | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.DataFrame.mad | pandas/core/generic.py | 9997.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L9997
pandas.DataFrame.mean | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.DataFrame.median | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.DataFrame.skew | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.DataFrame.to_html | pandas/core/frame.py | 2206.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L2206
pandas.melt | pandas/core/reshape/melt.py | 21.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/reshape/melt.py#L21
pandas.describe_option | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.reset_option | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.get_option | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.set_option | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.core.groupby.GroupBy.head | pandas/core/groupby/groupby.py | 2039.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L2039
pandas.core.groupby.GroupBy.mean | pandas/core/groupby/groupby.py | 1082.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L1082
pandas.core.groupby.GroupBy.nth | pandas/core/groupby/groupby.py | 1530.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L1530
pandas.core.groupby.GroupBy.tail | pandas/core/groupby/groupby.py | 2067.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L2067
pandas.core.groupby.DataFrameGroupBy.corr | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.core.groupby.DataFrameGroupBy.cov | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.core.groupby.DataFrameGroupBy.diff | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
... | ... | ... | ...
pandas.Series.to_numpy | pandas/core/base.py | 866.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/base.py#L866
pandas.Series.agg | pandas/core/series.py | 3427.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3427
pandas.Series.aggregate | pandas/core/series.py | 3427.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3427
pandas.Series.cummax | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.Series.cummin | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.Series.cumprod | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.Series.cumsum | pandas/core/generic.py | 10986.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10986
pandas.Series.kurt | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Series.mad | pandas/core/generic.py | 9997.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L9997
pandas.Series.mean | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Series.median | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Series.skew | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Series.kurtosis | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Series.compound | pandas/core/generic.py | 10038.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10038
pandas.Series.dt.dayofweek | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.weekday | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_month_start | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_month_end | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_quarter_start | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_quarter_end | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_year_start | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_year_end | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.Series.dt.is_leap_year | pandas/core/accessor.py | 80.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L80
pandas.core.window.Rolling.aggregate | pandas/core/window.py | 1678.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L1678
pandas.core.window.Expanding.aggregate | pandas/core/window.py | 1956.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L1956
pandas.Series.ptp | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Panel.compound | pandas/core/generic.py | 10038.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10038
pandas.Panel.kurt | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Panel.kurtosis | pandas/core/generic.py | 10941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10941
pandas.Panel.mad | pandas/core/generic.py | 9997.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L9997


Todo:

* [ ]   get rid of the errors in the code base
* [ ]   update the `code_check.sh` script to take into account the `GL03` type of errors