Fix bug in get_unique_universes introduced by #425
Fix bug in get_unique_universes introduced by #425

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