PenaltyConstrainedSolver: use HypreParMatrix and preconditioner with BoomerAMG
PenaltyConstrainedSolver: use HypreParMatrix and preconditioner with BoomerAMG

Note this is no longer valgrind clean, there's some weird thing in
mfem::Add(HypreParMatrix, HypreParMatrix) that I can't track down.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem