Track-length Photon Heating Estimator

So as mentioned during the monthly it’d be nice to have a track-length estimator for photon heating.
(Motivation, better stats.)

How MCNP5/6 handles this (their heating number) is detailed here:
PDF page 122


Thanks for the post @yrrepy as well as the link to the relevant section of the MCNP manual. The reason we don’t do a tracklength tally with a heating number for photons, as described in the MCNP manual, is because that’s only valid if thick-target bremsstrahlung is off. The MCNP manual even states further down when talking about equivalence between different tally types that:

For the photon results to be identical, both electron transport and the thick-target bremsstrahlung
approximation (PHYS:P j 1) must be turned off.

To be honest, I don’t know how much the approximation in MCNP will affect accuracy and under what situations, but I think that’s something we’d want to understand before giving an option that gives potentially less accurate answers.