[bugfix] H nuclei are being double-counted in the Arepo frontend

[jzuhone]

PR Summary

yt computes "nuclei density" fields by summing up ion number density fields for the same nuclei with different ionization states. The Arepo frontend currently has "H", "H_p0", and "H_p1" all added to the species_names attribute in ArepoFieldInfo, which is then used to compute the set up the species fields and compute the fields for specific abundances.

The "H_p0" and "H_p1" do not belong in this list for two reasons:

1. Their respective associated fields are handled later on in the method ArepoFieldInfo.setup_gas_particle_fields, and do not need to be handled by setup_species_fields.
2. Putting them in species_names results in the number of hydrogen nuclei being double-counted

This PR removes those ionization species from the species_names attribute and also removes an alias between H_nuclei_density and H_number_density which was supposed to work (and maybe did at one point) but is no longer working. It is also unnecessary with the above fix.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[jzuhone]

@matthewturk or @neutrinoceros this is a fix which should be merged pretty quickly if possible

[neutrinoceros]

This seems sound. I'd like to get a deeper look at this tomorrow to make sure I understand what's going on. In particular I want to check that removing the alias does indeed not count as a breaking change, as you suggested.

[neutrinoceros]

So, I find that there's only one sample dataset that this can be tested with, namely "TNGHalo"
Running this script

import yt

ds = yt.load_sample("TNGHalo")

ad = ds.all_data()

print(ad["gas", "H_number_density"])
print(ad["gas", "H_nuclei_density"])

I get

...
[0.06318717 0.09067829 0.1182974  ... 0.02359772 0.00351616 0.06363773] cm**(-3)
[0.12637434 0.18135657 0.2365948  ... 0.04719544 0.00703232 0.12727546] cm**(-3)

I see that indeed, the "alias" field ("gas", "H_nuclei_density") is incorrectly aliased.
I can also confirm, with this minimal reproducer, that your patch fixes the bug. To my surprise, it also does break my script (I was expecting ("gas", "H_nuclei_density") to simply become unavailable), so this patch is indeed eligible for back porting to yt 4.1.x (yay!).
Could you please add a small test case ? it doesn't have to be much more complicated that my script, changing the print for some assert statements.

[jzuhone]

Grrr stupid python 3.9 test

[neutrinoceros]

(Soon we'll get rid of these with #4122)