Set software backend for matplotlib
Set software backend for matplotlib

This was causing jobs to fail on the CSDMS compute node on
blanca. (They would run on the login node.)

@mcflugen pointed out that this may be a hardware issue--the compute
nodes may vary. I'll try to isolate this and notify RC.

csdms/wmt-exe,csdms/wmt-exe,csdms/wmt-exe,csdms/wmt-exe