Use accessors in some internal methods in FEMContext
Use accessors in some internal methods in FEMContext

Fourth pass. I think FEMContext is about all set. Now
it's DiffContext's turn.

Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh,Mbewu/libmesh