Add ".extxyz" file suffix

[PicoCentauri]

As requested by @frostedoyster. .extxyz and .xyz are usually the same and can use the same readers for them.

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📚 Documentation preview 📚: https://metatensor-models--84.org.readthedocs.build/en/84/

[DavideTisi]

great I wanted to ask this feature.
is it possible to add a test for this?
even just change the name of one of the dataset we use to .extxyz

[DavideTisi]

great I wanted to ask this feature. is it possible to add a test for this? even just change the name of one of the dataset we use to .extxyz

I you think is not necessary for me it also fine

[PicoCentauri]

great I wanted to ask this feature. is it possible to add a test for this? even just change the name of one of the dataset we use to .extxyz

Yes I can do a symbolic link and test this.

[PicoCentauri]

I think that should be enough for testing this.

[Luthaf]

Not a comment for this PR, but long term it might make sense to invert the control flow here. Instead of guessing the reader from the file extension, do something like

structures:
    read_from: file.xyz
    reader: ase

or

structures:
    read_from: ase://file.xyz

this way we don't try to use ASE on CP2K xyz files, which are not quite compatible

[PicoCentauri]

Hmm yes, even though I like the second API I think we should go for the first one. Also this comes with less string manioulation if the user leaves the reader field empty.

[DavideTisi]

I like the first API more, I think it is easier to read.
So I agree with going with the first one

[PicoCentauri]

Okay I do an issue for this. Then we can keep track.