For tallying nuclides, search when no default_xs is specified. Closes #119 on github.
For tallying nuclides, search when no default_xs is specified. Closes #119 on github.

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