Fixes and Tests for PolarsMixin (#56)

* fix to _df_all and _df_concat

* adding right overload to concat

* change _df_filter to _df_get_masked_df + _df_all

* adding custom name to _df_groupby_cumcount

* adding tests for PolarsMixin

* fixes to PolarsMixins

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* adding index_cols to _df_join per abstract DataFrameMixin

* fixes to method logic

* testing with index > 1 and 1 value

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>