Get rid of union_grid_index module variable.
Get rid of union_grid_index module variable.

johnnyliu27/openmc,bhermanmit/openmc,wbinventor/openmc,wbinventor/openmc,paulromano/openmc,liangjg/openmc,mjlong/openmc,smharper/openmc,mit-crpg/openmc,liangjg/openmc,wbinventor/openmc,mit-crpg/openmc,kellyrowland/openmc,paulromano/openmc,liangjg/openmc,shikhar413/openmc,smharper/openmc,amandalund/openmc,amandalund/openmc,amandalund/openmc,smharper/openmc,johnnyliu27/openmc,paulromano/openmc,mjlong/openmc,wbinventor/openmc,shikhar413/openmc,kellyrowland/openmc,walshjon/openmc,walshjon/openmc,samuelshaner/openmc,paulromano/openmc,liangjg/openmc,mit-crpg/openmc,amandalund/openmc,mit-crpg/openmc,smharper/openmc,walshjon/openmc,walshjon/openmc,shikhar413/openmc,shikhar413/openmc,bhermanmit/openmc,samuelshaner/openmc,johnnyliu27/openmc,johnnyliu27/openmc,samuelshaner/openmc,samuelshaner/openmc