Core-skipping
- Imported the core-skipping algorithm implemented by Jeremy in the dev-harley branch. This algorithm is based on Smith et al (2015).
- Moved most of the computation and calls from module_photon to module_gas_composition. This makes it more encapsulated into module_gas_composition with HI, and less apparent in module_photon.
- Also corrected a bug in the computation of tau_cell used for the core-skipping.
- And finally ported all the changes (arguments added in subroutines gas_scatter and gas_get_scatter_flag) in all gas_composition modules (to get the code compile).