Fix indexing bug in continuous S(a,b) treatment
Fix indexing bug in continuous S(a,b) treatment

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