Remove groups_mapping parameter

[edwardwliu]

Only uses single Panda series to represent groups and removes groups from initial forest instantiation. Fixes the implementation of groups with reproducibility against the R package. Also resolves tree representation of averaging info.

[petrovicboban]

I would rebase this and set last commit "update styling" as fixup for previous one, so we don't have small style fix commits. @edwardwliu

[edwardwliu]

FYI ended up with odd history, but will keep squashing style checks in mind going forward.

[theo-s]

Sounds good, thanks for removing this