Added docstrings for node grouping fxns
Added docstrings for node grouping fxns

Added docstrings to the node grouping fxns so it's easier for me and others
to figure out how exactly this works when it's revisited in the future.

johnbachman/indra,sorgerlab/belpy,bgyori/indra,johnbachman/belpy,sorgerlab/indra,johnbachman/indra,pvtodorov/indra,pvtodorov/indra,bgyori/indra,sorgerlab/indra,sorgerlab/belpy,bgyori/indra,pvtodorov/indra,johnbachman/belpy,pvtodorov/indra,sorgerlab/belpy,johnbachman/indra,sorgerlab/indra,johnbachman/belpy