Added check to see if photon crosses PI edge along path element ds and adjust kappa accordingly
changed frequency recorded in heating to be average of threshold + max along ds
Cleaned up and commented
corrected version
Corrected error when calculating frac_path in radiation
corrected frequency to freq_xs
Merge pull request #77 from jhmatthews/edge_hotfix

check if a photoionization edge is crossed (from Doppler shifts) along path element ds