Fix validation error type `GL04` and check in CI
#### Problem description
In order to have a continuous check by the CI on validation error `GL04` (Private classes should not be mentioned in public docstrings), fixing them in the code base enables the addition to the CI for automated testing in the future.

`GL04` has just 32 occurrences (see [overview of the errors](https://gist.github.com/198cfa552c727d71bd7360fda66fdbb4)):

method | file | file_line | github_link
-- | -- | -- | --
pandas.core.resample.Resampler.get_group | pandas/core/groupby/groupby.py | 626.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L626
pandas.core.groupby.GroupBy.get_group | pandas/core/groupby/groupby.py | 626.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/groupby/groupby.py#L626
pandas.core.groupby.DataFrameGroupBy.tshift | None | NaN | https://github.com/pandas-dev/pandas/blob/master/None#LNone
pandas.Series.pipe | pandas/core/generic.py | 4924.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L4924
pandas.Series.where | pandas/core/generic.py | 8820.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8820
pandas.Series.mask | pandas/core/generic.py | 8841.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8841
pandas.Series.first_valid_index | pandas/core/generic.py | 10235.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10235
pandas.Series.last_valid_index | pandas/core/generic.py | 10240.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10240
pandas.Series.tshift | pandas/core/generic.py | 8991.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8991
pandas.Series.to_dense | pandas/core/generic.py | 1920.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L1920
pandas.Index.searchsorted | pandas/core/base.py | 1497.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/base.py#L1497
pandas.DataFrame.where | pandas/core/generic.py | 8820.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8820
pandas.DataFrame.mask | pandas/core/generic.py | 8841.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8841
pandas.DataFrame.pipe | pandas/core/generic.py | 4924.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L4924
pandas.DataFrame.tshift | pandas/core/generic.py | 8991.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8991
pandas.DataFrame.first_valid_index | pandas/core/generic.py | 10235.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10235
pandas.DataFrame.last_valid_index | pandas/core/generic.py | 10240.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10240
pandas.DataFrame.to_dense | pandas/core/generic.py | 1920.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L1920
pandas.Panel.axes | pandas/core/generic.py | 441.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L441
pandas.Panel.isna | pandas/core/generic.py | 7084.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7084
pandas.Panel.notna | pandas/core/generic.py | 7152.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7152
pandas.Panel.rename | pandas/core/panel.py | 1252.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L1252
pandas.Panel.first_valid_index | pandas/core/generic.py | 10235.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10235
pandas.Panel.isnull | pandas/core/generic.py | 7088.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7088
pandas.Panel.last_valid_index | pandas/core/generic.py | 10240.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10240
pandas.Panel.mask | pandas/core/generic.py | 8841.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8841
pandas.Panel.notnull | pandas/core/generic.py | 7156.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7156
pandas.Panel.pipe | pandas/core/generic.py | 4924.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L4924
pandas.Panel.replace | pandas/core/generic.py | 6444.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6444
pandas.Panel.to_dense | pandas/core/generic.py | 1920.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L1920
pandas.Panel.transform | pandas/core/generic.py | 10175.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10175
pandas.Panel.where | pandas/core/generic.py | 8820.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L8820

Todo:
* [ ]  get rid of the errors in the code base
* [ ]  update the `code_check.sh` script to take into account the `GL04` type of errors