Ctxcore

* Remove unused RankingsDatabase classes and conversion functions.
* Add CisTargetDatabase class for reading rankings/scores for
regions/genes from a cisTarget scores or rankings database
(only Feather v2 format), which requires pyarrow >= 8.0.0
instead of <=0.17.1 needed for the old FeatherRankingDatabase
class.
* Update FeatherRankingDatabase to use CisTargetDatabase to read
cisTarget databases.
* Reformat code and update type annotations.

* Add support for reading rankings databases created by
create_cisTarget_databases as the index column in the
Feather file is "motifs" or "tracks" instead of "features".

* Project created.