Fixed -npstring command line argument in g_tune_pme

-npstring <str> is used to let g_tune_pme know how it should specify
the number of MPI processes to the mpirun command (with -np (default) or -n ).
You could provide it, but it actually -np was used in any case.

Change-Id: Ibb807b492dfb05c67628c18b7073092aa983f770