Implement function to obtain charge state distribution

[lemmatum]

See this discussion in PlasmaPy.

Saha-Boltzmann may be a good starting point. This should be temperature and density sensitive. We can then test it against NIST.

CHIANTI and fiasco have functions for this, but for diffuse, high-temperature plasmas (e.g. solar corona), where LTE doesn't hold. We may be able to call those for certain non-LTE distributions, but we should implement our own more general methods to cover wider range of plasmas.

[lemmatum]

I think I've got most of the pieces to implement a version of this using Saha-Boltzmann equations.