Add MPI communicator option to the MUMPS solver interface

[chapman39]

This PR adds an MPI communicator option to the MUMPS solver interface. The motivation for this came from wanting to use MUMPS with MPI_COMM_SELF.

The project that I'm working on, internal to LLNL, runs in parallel, but runs MUMPS serially. In order to achieve this, we configure MUMPS with MPI, but set --disable-mpiinit as described here under "MUMPS Linear Solver".

We use Spack to install our third party libraries, including Ipopt and MUMPS. This means we are not using the recommended ThirdParty-Mumps. Figured I would share for awareness (our package.py is slightly different): https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/ipopt/package.py.

Before the changes in this PR, our tests were hanging when run with multiple MPI tasks.

[CLAassistant]

All committers have signed the CLA.