minor change to add an inline version of Mgxs::get_xs just like @gridley did for mg.get_*_xs
minor change to add an inline version of Mgxs::get_xs just like @gridley did for mg.get_*_xs

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