Fix fetch_by_source_stable_id, fetch_by_SeqMember.

Fix to get protein families view working. Fixes deprecated for e79 methods, but they're not supposed to stop working quite yet!

1. fetch_by_SeqMember should return only the unique SeqMember, not an arrayref, assuming its description is correct and based on how it is used.

2. fetch_by_Member_source_stable_id now prospectively tries calling >fetch_by_source_stable_id on GeneMemberAdaptor first, if that fails does the same on SeqMemberAdaptor. Sadly GeneMemberAdaptor failed too hard, so you could never retrieve SeqMembers with this method.