Add option to fill all species when computing PowerSpectrum

[PicoCentauri]

Possible fix for #213.

This adds the option fill_species_neighbor to the compute method of PowerSpectrum. Setting this to True changes keys_to_move when performing keys_to_properties in such a way that it contains all species of the provided systems.

I am not super happy with the name of the parameter but I think the it should be part of the the compute method.

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📚 Documentation preview 📚: https://rascaline--214.org.readthedocs.build/en/214/

[github-actions]

Here is a pre-built version of the code in this pull request: wheels.zip, you can install it locally by unzipping wheels.zip and using pip to install the file matching your system

[ceriottm]

I think the ideal implementation would be something that has an option neighbor_species that defaults to None (and then automatically fetches them from the inputs) or a list of labels or numbers (and then makes sure that all the missing ones are padded with zeros.

[Luthaf]

I think I'd be fine with this being the only behavior: we are not introducing too much zeros since we are moving the two spherical expansions separately, so we could just always do this and not have an extra option.

[PicoCentauri]

Okay, even better if we do not overload the parameters with more and more options.

[Luthaf]

I think the ideal implementation would be something that has an option neighbor_species that defaults to None (and then automatically fetches them from the inputs) or a list of labels or numbers (and then makes sure that all the missing ones are padded with zeros.

That could work as well, but I don't love that we are introducing a new set of parameters just for this. This feels very similar to the job done by selected_properties. Implementing full support for selected_properties in Python would be relatively hard, but we should also consider it!

I am not super happy with the name of the parameter but I think the it should be part of the the compute method.

The issue with making it part of the compute method is that we now have different set of options for Rust calculators and this Python calculator, which I don't love. I agree it feels much more natural to have it here though!