Asymptotic preserving MOCMC samples for pure scattering

[brryan]

Recording a discussion with @jdolence to be addressed in the future:

There is an issue with asymptotic preserving transport of sample intensities in MOCMC for a purely scattering medium. In this case scattering moves each particle's intensity in the cell towards the cell's angle-averaged value. However, this is done after the transport step -- if optical depths are infinite, the samples will nonetheless transport their full intensities across a solution to then be averaged with neighbors. The result is that at late time the initial radiation energy density profile in the moments will be completely averaged out across the domain in the samples. This can be missed because (1) when scattering is large the samples will still correctly produce Pij = diag(1/3) everywhere (2) absorption/emission still act to drive the samples towards the moment energy densities.

This can probably be fixed by appropriate use (interpolation towards?) the angle-averaged intensities prior to transport for each step when calculating the effective scattering emissivity.