Fix validation error type `RT05` and check in CI
#### Problem description
In order to have a continuous check by the CI on validation error `RT05` (Return value description should finish with "."), fixing them in the code base enables the addition to the CI for automated testing in the future.

`RT05` has just 51 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.core.resample.Resampler.interpolate | pandas/core/resample.py | 762.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/resample.py#L762
pandas.plotting.bootstrap_plot | pandas/plotting/_misc.py | 363.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_misc.py#L363
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.clip | pandas/core/generic.py | 7208.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7208
pandas.Series.unique | pandas/core/series.py | 1612.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L1612
pandas.Series.align | pandas/core/series.py | 3652.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3652
pandas.Series.interpolate | pandas/core/generic.py | 6784.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6784
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.strftime | pandas/core/accessor.py | 90.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L90
pandas.tseries.frequencies.to_offset | pandas/tseries/frequencies.py | 59.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/tseries/frequencies.py#L59
pandas.Index.reindex | pandas/core/indexes/base.py | 3088.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L3088
pandas.Index.isna | pandas/core/indexes/base.py | 1815.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L1815
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.Index.get_indexer_non_unique | pandas/core/indexes/base.py | 4428.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L4428
pandas.MultiIndex.sortlevel | pandas/core/indexes/multi.py | 2038.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/multi.py#L2038
pandas.MultiIndex.swaplevel | pandas/core/indexes/multi.py | 1941.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/multi.py#L1941
pandas.DatetimeIndex.strftime | pandas/core/indexes/datetimelike.py | 47.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/datetimelike.py#L47
pandas.PeriodIndex.strftime | pandas/core/accessor.py | 90.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/accessor.py#L90
pandas.DataFrame.get_ftype_counts | pandas/core/generic.py | 5419.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L5419
pandas.DataFrame.get_values | pandas/core/generic.py | 5342.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L5342
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.clip | pandas/core/generic.py | 7208.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7208
pandas.DataFrame.align | pandas/core/frame.py | 3784.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L3784
pandas.DataFrame.interpolate | pandas/core/generic.py | 6784.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6784
pandas.DataFrame.asof | pandas/core/generic.py | 6855.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.DataFrame.plot.area | pandas/plotting/_core.py | 3312.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_core.py#L3312
pandas.DataFrame.boxplot | pandas/plotting/_core.py | 2243.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_core.py#L2243
pandas.Panel.get_ftype_counts | pandas/core/generic.py | 5419.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L5419
pandas.Panel.set_value | pandas/core/panel.py | 522.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L522
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.Panel.clip | pandas/core/generic.py | 7208.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L7208
pandas.Categorical.__array__ | pandas/core/arrays/categorical.py | 1286.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/categorical.py#L1286
pandas.unique | pandas/core/algorithms.py | 278.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/algorithms.py#L278
pandas.wide_to_long | pandas/core/reshape/melt.py | 181.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/reshape/melt.py#L181
pandas.to_numeric | pandas/core/tools/numeric.py | 14.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/tools/numeric.py#L14
pandas.core.window.Rolling.kurt | pandas/core/window.py | 1777.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L1777
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.kurt | pandas/core/window.py | 2050.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L2050
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.render | pandas/io/formats/style.py | 421.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/style.py#L421
pandas.Series.get_ftype_counts | pandas/core/generic.py | 5419.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L5419
pandas.Series.set_value | pandas/core/series.py | 1190.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L1190
pandas.DataFrame.set_value | pandas/core/frame.py | 2782.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L2782
pandas.Panel.asof | pandas/core/generic.py | 6855.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.Panel.get_values | pandas/core/generic.py | 5342.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L5342
pandas.Panel.interpolate | pandas/core/generic.py | 6784.0 | https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6784

Todo:
* [ ]  get rid of the errors in the code base
* [ ]  update the `code_check.sh` script to take into account the `RT05` type of errors