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?

[wtbarnes]

There are properties related to the older treatment of the level-resolved ionization and recombination rate data (cilvl and reclvl files, respectively) that I've called _level_resolved_ionization_rate and _level_resolved_recombination_rate that have an unfortunate namespace clash with the functionality I'm adding here.

fiasco/fiasco/ions.py

Lines 997 to 1019 in 43ad865

	@cached_property
	@needs_dataset('cilvl')
	@u.quantity_input
	def _level_resolved_ionization_rate(self):
	ionization_rates = self._level_resolved_rates_interpolation(
	self._cilvl['temperature'],
	self._cilvl['ionization_rate'],
	fill_below='extrapolate',
	fill_above=0.0,
	)
	return self._cilvl['upper_level'], ionization_rates
	@cached_property
	@needs_dataset('reclvl')
	@u.quantity_input
	def _level_resolved_recombination_rate(self):
	recombination_rates = self._level_resolved_rates_interpolation(
	self._reclvl['temperature'],
	self._reclvl['recombination_rate'],
	fill_below=0.0,
	fill_above='extrapolate',
	)
	return self._reclvl['upper_level'], recombination_rates

These should just be folded into the _population_correction method as that is the only place they are used anyway.

fiasco/fiasco/ions.py

Line 1022 in 43ad865

def _population_correction(self, population, density, rate_matrix):

These are not meant to be used on their own and should not be confused with the new advanced model data.