Allow material='void' in RELAX NG schema.
Allow material='void' in RELAX NG schema.

shikhar413/openmc,mjlong/openmc,paulromano/openmc,walshjon/openmc,wbinventor/openmc,keadyk/openmc_mg_prepush,mit-crpg/openmc,bhermanmit/openmc,nhorelik/openmc,johnnyliu27/openmc,samuelshaner/openmc,shikhar413/openmc,smharper/openmc,liangjg/openmc,paulromano/openmc,keadyk/openmc_mg_prepush,mit-crpg/openmc,mjlong/openmc,amandalund/openmc,nhorelik/openmc,sxds/opemmc,sxds/opemmc,samuelshaner/openmc,lilulu/openmc,amandalund/openmc,shikhar413/openmc,mit-crpg/openmc,walshjon/openmc,samuelshaner/openmc,liangjg/openmc,wbinventor/openmc,keadyk/openmc_mg_prepush,shenqicang/openmc,johnnyliu27/openmc,johnnyliu27/openmc,shenqicang/openmc,walshjon/openmc,lilulu/openmc,liangjg/openmc,shikhar413/openmc,bhermanmit/openmc,kellyrowland/openmc,bhermanmit/cdash,wbinventor/openmc,mit-crpg/openmc,amandalund/openmc,paulromano/openmc,smharper/openmc,johnnyliu27/openmc,kellyrowland/openmc,liangjg/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,walshjon/openmc,samuelshaner/openmc,wbinventor/openmc,lilulu/openmc,amandalund/openmc