:propn-extended-fams-idxs now propn-extended-fams-ids

i.e. the seqs of things associated with a propn id are now ids,
because that is that (currently) the functions called by receive-propn
need.  Reluctant to do it all in terms of indexes, which are hard to
investigate.
modified docstring for ->PropnNet
trivial mod of docstring
added commented-out function def mapseqs
Added comment.
Revised receive-propn to iterate through extended families

i.e. if a propn is newly received, then there may be other propns
that can now participate in analogies because it's there.  So we
need to try to form analogies using all of the propns in extended
families of which the sent propn is a member.