Fixes to indexing and masking. Also updating material id attribute to id_.
Fixes to indexing and masking. Also updating material id attribute to id_.

wbinventor/openmc,paulromano/openmc,liangjg/openmc,wbinventor/openmc,amandalund/openmc,paulromano/openmc,walshjon/openmc,amandalund/openmc,liangjg/openmc,smharper/openmc,mit-crpg/openmc,walshjon/openmc,paulromano/openmc,mit-crpg/openmc,shikhar413/openmc,johnnyliu27/openmc,amandalund/openmc,wbinventor/openmc,walshjon/openmc,shikhar413/openmc,smharper/openmc,paulromano/openmc,mit-crpg/openmc,liangjg/openmc,walshjon/openmc,johnnyliu27/openmc,johnnyliu27/openmc,shikhar413/openmc,shikhar413/openmc,mit-crpg/openmc,liangjg/openmc,wbinventor/openmc,smharper/openmc,amandalund/openmc,johnnyliu27/openmc,smharper/openmc