Help clients find MPI-enabled GROMACS

Client CMake scripts have to take unconventional steps to find MPI-enabled GROMACS.

Currently, MPI-enabled GROMACS is found by defining GROMACS_SUFFIX before performing find_package().

Modern CMake allows us to search for "components" or provide other constraints on the features of a package we search for, so we have several options to update the semantics of find_package(GROMACS...).

There does not appear to be any fall-back logic to allow find_package(GROMACS) to simply return any GROMACS installation found.