Make structure adapters infer species from species_at_sites when missing

[ml-evs]

I keep running into this issue where implementations do not return species by default, because their species dictionaries are extremely simple (i.e., single chemical symbol per species, no disorder, and the name follows the element symbol).

This PR allows the ASE, AiiDA and pymatgen structure adapters to infer this simple structure from just species_at_sites when species is missing/None.

[ml-evs]

I need this for a demo of the materials resource registry coming up soon, so will merge and release in a few days if there are no complaints.

[codecov]

Codecov Report

Merging #1103 (71136d7) into master (8dcde4d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1103      +/-   ##
==========================================
+ Coverage   92.30%   92.33%   +0.02%     
==========================================
  Files          68       68              
  Lines        3834     3849      +15     
==========================================
+ Hits         3539     3554      +15     
  Misses        295      295              

Flag	Coverage Δ
project	92.33% <100.00%> (+0.02%)	⬆️
validator	92.33% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/adapters/structures/aiida.py	100.00% <100.00%> (ø)
optimade/adapters/structures/ase.py	97.29% <100.00%> (+0.32%)	⬆️
optimade/adapters/structures/pymatgen.py	100.00% <100.00%> (ø)
optimade/adapters/structures/utils.py	81.30% <100.00%> (+0.46%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8dcde4d...71136d7. Read the comment docs.

[JPBergsma]

Looks good to me.

[ml-evs]

Thanks @JPBergsma! I think @CasperWA is the main user of these adapters within other projects, so I'll just wait for the OK from him too.

[ml-evs]

I've had a look at the code that uses these adapters and I'm fairly certain that these changes are fine; will merge and release.