science/nwchem: Add options to choose MPI implementation

MPICH is currently broken in the runtime, see https://github.com/nwchemgit/nwchem/issues/463
It works with OPENMPI=yes but thi can't be made default because
math/scalapack and devel/ga need to have the same choice of MPI
but dependencies of math/scalapack fail with OPENMPI=yes.