Allow table grouping across multiple semantic table types

[jwdebelius]

Currently, group only accepts a FeatureTable[Frequency] semantic type. It makes logical and practical sense to extend it to other semantic table types, like relative abundance and compositional representations. Perhaps the allowed semantic types should or could be extended in a manner similar to the one used for rename-ids or filter-samples, although one question to consider is whether it makes sense to allow grouping on the FeatureTable[PresenceAbsence] semantic type, or if it should be constrained to abundance-based values.

This came up in this thread in the forum.