Re-integrate saving the photon path in scattering calculations for use in continuous absorption

[karllark]

The calculation of the photon path is repeated in the forced first scattering code. Once to determine the maximum optical depth along the line-of-sight and a 2nd time to move the photon to the picked scattering site. Code was written by Ka-Hei to do this, but it only worked for a single grid (no hierarchical grids). Need to re-integrate or develop code to do this. A similar issue exists for the next_scatter/scatter_photon functions. Also may want to cache this calculation for the stellar classification in the case of discrete sources.