Added missing fixed source subroutine
Added missing fixed source subroutine

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