Implement cosmic ray attenuation

[psharda]

At some point, we might want to think about implementing cosmic ray attenuation within the ISM chemistry networks. The simplest way is to attenuate the cosmic ray ionization rate as a function of the gas column density (e.g, following Appendix E of Padovani et al. 2020). But maybe @markkrumholz has better ideas. Creating an issue here for future reference.