Unable to build `opensn` with `openmpi`

[andrsd]

When using openmpi the code does not build. The error message seen is:

/path/mpicpp-lite/include/mpicpp-lite/impl/Group.h:125:19: error: incompatible integer to pointer conversion assigning to 'MPI_Group' (aka 'ompi_group_t *') from '(unnamed enum at /path/mpicpp-lite/include/mpicpp-lite/impl/Enums.h:5:1)'
    this->group = UNDEFINED;
                  ^~~~~~~~~

This is a problem in mpicpp-lite, see issue.

[andrsd]

A follow-up on the mpicpp-lite development. I added testing against openmpi. All changes that bubble through the upstream repo are now checked against mpich and openmpi which should give us greater confidence that we won't break OpenSn when we pull those changes here.