Change some domdec data members to RVec or IVec

Change some domdec data members to RVec or IVec

Where a trivial change from rvec/ivec to RVec/IVec is possible,
that change has been made. The data member gmx_domdec_t.nc is
renamed to numCells to facilitate completing the todo about
changing this member to IVec. Some dead code is also removed
from the domdec module.

Change-Id: I1ef78c5ec198c60a1dd40a57ee5f87feb48bf490