sample_restraint can't find gromacs_mpi-config.cmake

[eirrgang]

If GROMACS is built with MPI, the cmake config file has a different name (unless GROMACS_SUFFIX is overridden by the user at GROMACS configure time) and the user then has to specify -DGROMACS_SUFFIX="_mpi" on the CMake command-line for sample_restraint, which is an arcane imposition on our users.

My expectation is that we will soon more fully separate the interface necessary for the restraint framework so that we don't need the libgromacs target at all (just Gromacs::gmxapi) in CMake. If that doesn't happen by the time #50 is resolved, we must resolve this issue by

• making FindGROMACS.cmake smarter
• making gmxapi_config.cmake smarter
• making sample_restraint/CMakeLists.txt smarter (not ideal)
• return to the premise and obviate the libgromacs target dependency before merging the solution to enable MPI domain decomposition with plugins #50