Added reference to CASMO-5 for factory method to generate common energy group structures
Added reference to CASMO-5 for factory method to generate common energy group structures

mit-crpg/openmc,shikhar413/openmc,liangjg/openmc,liangjg/openmc,shikhar413/openmc,smharper/openmc,liangjg/openmc,amandalund/openmc,amandalund/openmc,paulromano/openmc,smharper/openmc,walshjon/openmc,wbinventor/openmc,smharper/openmc,shikhar413/openmc,wbinventor/openmc,liangjg/openmc,smharper/openmc,johnnyliu27/openmc,johnnyliu27/openmc,mit-crpg/openmc,johnnyliu27/openmc,wbinventor/openmc,paulromano/openmc,amandalund/openmc,paulromano/openmc,amandalund/openmc,wbinventor/openmc,walshjon/openmc,mit-crpg/openmc,walshjon/openmc,mit-crpg/openmc,shikhar413/openmc,paulromano/openmc,walshjon/openmc,johnnyliu27/openmc