Minor fixes to 2group solver
Minor fixes to 2group solver

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