Change default MPI implementation from openmpi to mvapich on ARC4

[leifdenby]

@eers1 noticed openmpi compiled version of monc crashes

[rwnsansom]

Simulation would pause at the point of sending diagnostics to I/O server until it terminated at walltime termination with the follow error in the output file:

No diagnostic_files produced.

Fixed by adding
"module switch openmpi mvapich2"
to compile file (monc_compile_arc.sh) and job submission file.

Also uncommenting the follow lines in the job submission file:
"# MVAPICH2 RUNTIME VARIABLES
MONC_THREAD_MULTIPLE=0
MV2_ENABLE_AFFINITY=0
MV2_SHOW_CPU_BINDING=1
MV2_USE_THREAD_WARNING=0
export MONC_THREAD_MULTIPLE MV2_ENABLE_AFFINITY MV2_SHOW_CPU_BINDING
MV2_USE_THREAD_WARNING"

[cemac-ccs]

PR #47 made to address this issue