Fix several bugs in continuous S(a,b) treatment
Fix several bugs in continuous S(a,b) treatment

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