Fixed subtle bug with S(a,b) cross section calculation.
Fixed subtle bug with S(a,b) cross section calculation.

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