Make ionization and recombination components functions, optionally return level-resolved rates

[wtbarnes]

Fixes #384
Fixes #385
Fixes #415
Fixes #416

Remaining TODOs

• Return level-resolved ionization rates
• Decide where to return the density-dependent advanced model rates
• Add function for selecting a model atmosphere
• Add methods for calculating CT ionization and recombination rates
• Allow Element.ionization_equilibrium to accept a density and atmosphere model.

[wtbarnes]

Some working notes for myself

• Should have separate methods for charge transfer ionization/recombination
• These can be added into total for ionization/recombination if model atmosphere is considered
• Should ionization/recombination rate methods be able to return the advanced model rates?
• These have the downside of needing to repeatedly calculate the level populations...
• If not, then are they just all calculated in Element.ionization_equilibrium?