imported exp, erf, pi funtion from math package
imported exp, erf, pi funtion from math package

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