converted isotopes from iterator to list for python 3 compatibility
converted isotopes from iterator to list for python 3 compatibility

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