Fix validation error type `RT04` and check in CI
#### Problem description
In order to have a continuous check by the CI on validation error `RT04` (Return value description should start with a capital letter), fixing them in the code base enables the addition to the CI for automated testing in the future.

`RT04` has just 33 occurrences (see [overview of the errors](https://gist.github.com/198cfa552c727d71bd7360fda66fdbb4)):

method | file | file_line | github_link
-- | -- | -- | --
pandas.core.resample.Resampler.apply | pandas/core/resample.py | 258.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/resample.py#L258
pandas.core.resample.Resampler.aggregate | pandas/core/resample.py | 258.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/resample.py#L258
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.quantile | pandas/core/series.py | 2004.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L2004
pandas.Series.asof | pandas/core/generic.py | 6855.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.Series.dt.to_pydatetime | pandas/core/indexes/accessors.py | 129.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/accessors.py#L129
pandas.Series.dt.to_pytimedelta | pandas/core/indexes/accessors.py | 199.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/accessors.py#L199
pandas.Index.shift | pandas/core/indexes/base.py | 4242.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L4242
pandas.MultiIndex.get_level_values | pandas/core/indexes/multi.py | 1380.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/multi.py#L1380
pandas.DataFrame.agg | pandas/core/frame.py | 6295.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6295
pandas.DataFrame.aggregate | pandas/core/frame.py | 6295.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6295
pandas.DataFrame.quantile | pandas/core/frame.py | 7717.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L7717
pandas.DataFrame.asof | pandas/core/generic.py | 6855.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.Panel.major_xs | pandas/core/panel.py | 793.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L793
pandas.Panel.minor_xs | pandas/core/panel.py | 817.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L817
pandas.api.types.is_categorical_dtype | pandas/core/dtypes/common.py | 572.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L572
pandas.api.types.is_datetime64_dtype | pandas/core/dtypes/common.py | 403.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L403
pandas.api.types.is_datetime64tz_dtype | pandas/core/dtypes/common.py | 434.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L434
pandas.api.types.is_extension_type | pandas/core/dtypes/common.py | 1643.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L1643
pandas.api.types.is_integer_dtype | pandas/core/dtypes/common.py | 868.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L868
pandas.api.types.is_interval_dtype | pandas/core/dtypes/common.py | 536.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L536
pandas.api.types.is_signed_integer_dtype | pandas/core/dtypes/common.py | 923.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L923
pandas.api.types.is_timedelta64_dtype | pandas/core/dtypes/common.py | 472.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L472
pandas.api.types.is_timedelta64_ns_dtype | pandas/core/dtypes/common.py | 1167.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L1167
pandas.api.types.is_unsigned_integer_dtype | pandas/core/dtypes/common.py | 980.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L980
pandas.api.types.is_datetimetz | pandas/core/dtypes/common.py | 294.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L294
pandas.unique | pandas/core/algorithms.py | 278.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/algorithms.py#L278
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.io.formats.style.Styler.from_custom_template | pandas/io/formats/style.py | 1210.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/style.py#L1210
pandas.io.formats.style.Styler.render | pandas/io/formats/style.py | 421.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/style.py#L421
pandas.Panel.asof | pandas/core/generic.py | 6855.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855

Todo:
* [ ]  get rid of the errors in the code base
* [ ]  update the `code_check.sh` script to take into account the `RT04` type of errors