Change the order of samples in spherical exansion by pair

[Luthaf]

This is a follow-up to #147.

Previously, the samples would look like this

structure, pair_id, first_atom, second_atom
   0          1         1           0
   0          1         0           1
   0          2         1           2
   0          2         2           1
   1          1         1           0
   1          1         0           1
                ...
   0         -1         0           0
   0         -1         1           1
   0         -1         2           2
   1         -1         0           0
   1         -1         1           1

i.e; with the "self" pairs at the end of the samples, meaning there could be a jump in the "structure" sample. This changes it to group all structures, placing the "self" pairs first inside the samples for a given structure.

[github-actions[bot]]

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

[github-actions[bot]]

The documentation for this PR is (or will soon be) available on readthedocs: https://rascaline--151.org.readthedocs.build/en/151/

[ceriottm]

Works as advertised!