added capability for more than 2 groups
added capability for more than 2 groups

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