added note about subclass model_type_ var setting
added note about subclass model_type_ var setting

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