Fix calculation of Material.density_gpcc_
Fix calculation of Material.density_gpcc_

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