Reimplement GammaHeatSource with clearer functionality #322
Description
GammaHeatSource currently takes in a average_fission_heat input parameter that should be the volume-averaged heat source in the graphite/non-fuel region. Users may unknowingly (and wrongly) provide average fission heat from the AverageFissionHeat postprocessor.
This issue can be closed when GammaHeatSource is refactored by renaming existing input parameters or by changing the implementation with the goal of improved user experience.