Issue warning if mpiexec/mpirun not detected and MPI logic used

[Grzyb33k]

No description provided.

[Grzyb33k]

Note that the env var name seems MPICH specific. To support OpenMPI, the OMPI_COMM_WORLD_SIZE would work:

$ mpiexec -n 1 printenv | grep SIZE
OMPI_UNIVERSE_SIZE=6
OMPI_COMM_WORLD_SIZE=1
OMPI_COMM_WORLD_LOCAL_SIZE=1