New formula for MUMPS.

MUMPS is a sequential or parallel solver for symmetric linear systems.

Technically, the parallel version of MUMPS can also use ParMETIS. However, it requires METIS v4 and the ParMETIS available in Homebrew/Science uses the METIS v5 API. I'm told METIS 5 support will be part of the next release of MUMPS. We can add ParMETIS as an optional dependency at that point.