Use SLURM_* vars when no MPI_LOCAL* vars
Created by: aaronknister
The environment variables MPI_LOCALNRANKS and MPI_LOCALRANKID don't appear to be native to SLURM. So as to not break sites alread using likwid that may have these MPI_LOCAL* vars use the SLURM_* equivalents only if the MPI_LOCAL* vars are unavailable.