Added reference for calculating percentiles of standard normal.
Added reference for calculating percentiles of standard normal.

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