Working code for generating phtn_src_total with a non-scd mesh. Needs further refactoring for cleanliness/minimization of code redundancies.
Allow gammas file specification in r2s.cfg.
Minor fix for robustness.
Cleanup; pass vols from calc_ method
Fix tests for changes to write_gammas and mcnp_n2p scripts.
Added optional specification of alternative location for gammas file.