Implement simple chemical networks
Needed for testing purposes and for toy models. The following networks should be created:
- Nelson & Langer (1999) original network with H2 chemistry (http://iopscience.iop.org/article/10.1086/307823/pdf)
- Gong et al. (2017) updated network for CO chemistry (https://arxiv.org/pdf/1610.09023v1.pdf)
- Szűcs et al. (2014) 12C and 13C chemical fractionation network (implement isotopologue species, add fractionation reaction)