Fix 300 K data in cross_sections_nndc.xml and add missing metastable tags.
Fix 300 K data in cross_sections_nndc.xml and add missing metastable tags.

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