applied katy's atom fraction nan patch with slight implementation tweaks, also included a guard in the mass fraction function
applied katy's atom fraction nan patch with slight implementation tweaks, also included a guard in the mass fraction function

mbmcgarry/cyclus,Baaaaam/cyclus,gidden/cyclus,hodger/cyclus,Baaaaam/cyclus,Baaaaam/cyclus,rwcarlsen/cyclus,gidden/cyclus,rwcarlsen/cyclus,mbmcgarry/cyclus,rwcarlsen/cyclus,hodger/cyclus,hodger/cyclus,hodger/cyclus,gidden/cyclus,gidden/cyclus,rwcarlsen/cyclus,mbmcgarry/cyclus,hodger/cyclus,mbmcgarry/cyclus