This commit adds the flag VTKm_ENABLE_GPU_MPI which when enable it
will use GPU AWARE MPI.

- This will only work with GPUs and MPI implementation that supports GPU
  AWARE MPI calls.
- Enabling VTKm_ENABLE_GPU_MPI without MPI/GPU support might results in
  errors when running VTK-m with DIY/MPI.
- Only the following tests can run with this feature if enabled:
  - UnitTestSerializationDataSet
  - UnitTestSerializationArrayHandle