Fixed nested hex_lattice error
Fixed nested hex_lattice error

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