Fix bug with <element> specifying xs attribute. Closes #269 on github.
Fix bug with <element> specifying xs attribute. Closes #269 on github.

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