Add an "overriding MSA" capability to headed compound rules

LT-2972, LT-2969, LT-2970, LT-2971

At this point we are only going to add inflection class.  We will not add
any of the other possible MoStemMsa properties until we have users asking
for them to be added.

Change-Id: Ie50785e2d40352878bc5b2bc93f90702befcc132