Update ionization rate calculation to include level-resolved rates

[wtbarnes]

In v11, the overall ionization rate can be computed as the sum of the level-resolved ionization rates weighted by the populations of those levels. The levels for which these ionization rates are provided are the so-called metastable levels. See section 2.1 and 2.4 of Dufresne et al. (2024) for more details.

One thing that will need to be decided is how to incorporate the density as these updated rates are now density-dependent since they depend on the level populations calculation. However, the ionization rate is currently implemented as a property such that it does not take any inputs. There are two options:

1. Turn ionization_rate into a method that takes a density
2. Make density an (optional) input to the Ion class

Even if using option 2, ionization_rate could still be a method that takes an argument saying whether to calculate the ionization rate using the level-resolved rates or not.

Before this can be completed, #374, #376, #377 need to be completed.

[wtbarnes]

#415 should be addressed first