Modify ion.n_levels

[jacobdparker]

Fixes #425

Adding logic to ion.n_levels to account for cases where more energy levels are described in the ion.elvlc than than are referred to in ion.wgfa or ion.scups. As of now, this only occurs for N 4.

[wtbarnes]

I find this logic a bit confusing. Is it not sufficient to just take the min between n_levels_elvlc and both the max level from the wgfa file and the max level from the scups file?

[wtbarnes]

Two additional requests:

1. Could you also add a test using "N 4" that checks that n_levels correctly returns the maximum number of levels in the wgfa file and not the number of levels in the elvlc file?
2. Could you add a note in the docstring saying that this property may not necessarily be equal to the number of levels in the elvlc file?

[jacobdparker]

I took the max between n_levels_scups and n_levels_wgfa assuming that if at least one file refered to an energy level, it should be included in the model. This only really matters for Ar 18, where if you took the minimum, level 25 would be excluded. Maybe it should be ...

I find two cases (updated from original post) where n_levels_elvlc is > n_levels_scups and n_levels_wgfa:

N 4
n_levels_elvlc=238
n_levels_wgfa=np.int64(136)
n_levels_scups=np.int64(136)

Ar 6
n_levels_elvlc=202
n_levels_wgfa=np.int64(155)
n_levels_scups=np.int64(155)

I find one case where n_levels_scups > n_levels_wgfa:
Ar 18
n_levels_elvlc=25
n_levels_wgfa=np.int64(24)
n_levels_scups=np.int64(25)

And a few cases where there is now scups or wgfa file:
Ti 22
n_levels_elvlc=64
n_levels_wgfa=0
n_levels_scups=0

Cr 24
n_levels_elvlc=64
n_levels_wgfa=0
n_levels_scups=0

Zn 30
n_levels_elvlc=64
n_levels_wgfa=0
n_levels_scups=0

[wtbarnes]

The cases where there are no scups or wgfa files are fine. In those cases, there is effectively no model and so you can't compute a contribution function anyway.

[wtbarnes]

Ok I'm now confused again about what the logic behind choosing the number of levels for the model is. This is what is in the IDL code:

https://github.com/chianti-atomic/chianti-idl/blob/121fd3caa160622ef08e1b9d13d3f930a76e1cfc/level_population/ch_load_ion_rates.pro#L308-L313

which is consistent with what we wrote above except for the inclusion of the autoionization rates. I think this is included to make sure that in the case of the two-ion model, those rates don't get cut off.

In practice, I don't find any ions where the max scups level is less than the autoionizing levels and the max autoionizing level isn't also equal to the max elvlc or wgfa level. In other words, However, I'm guessing we should still safeguard against this possibility and just implement the IDL logic here.

I'll push a commit to make this change.