Implemented initial fixed source mode in Python API
Implemented initial fixed source mode in Python API

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