Allow run_tests.py to work even without MPI installed
Allow run_tests.py to work even without MPI installed

samuelshaner/openmc,wbinventor/openmc,walshjon/openmc,mjlong/openmc,bhermanmit/openmc,amandalund/openmc,mit-crpg/openmc,smharper/openmc,shikhar413/openmc,mit-crpg/openmc,smharper/openmc,mit-crpg/openmc,walshjon/openmc,paulromano/openmc,kellyrowland/openmc,johnnyliu27/openmc,kellyrowland/openmc,smharper/openmc,amandalund/openmc,smharper/openmc,shikhar413/openmc,samuelshaner/openmc,paulromano/openmc,wbinventor/openmc,samuelshaner/openmc,bhermanmit/openmc,johnnyliu27/openmc,liangjg/openmc,johnnyliu27/openmc,amandalund/openmc,wbinventor/openmc,walshjon/openmc,johnnyliu27/openmc,amandalund/openmc,walshjon/openmc,liangjg/openmc,mit-crpg/openmc,liangjg/openmc,liangjg/openmc,paulromano/openmc,wbinventor/openmc,paulromano/openmc,shikhar413/openmc,samuelshaner/openmc,mjlong/openmc,shikhar413/openmc