Check for carbon in elements for Hill

[CasperWA]

Fixes #585.

If carbon is not present, the elements should be sorted alphabetically (including hydrogen).

This changes a test to the corrected definition of Hill notation.

[codecov]

Codecov Report

Merging #586 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
- Coverage   91.77%   91.77%   -0.01%     
==========================================
  Files          62       62              
  Lines        3222     3220       -2     
==========================================
- Hits         2957     2955       -2     
  Misses        265      265              

Flag	Coverage Δ
project	91.77% <100.00%> (-0.01%)	⬇️
validator	64.68% <0.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
optimade/models/structures.py	95.75% <100.00%> (-0.04%)	⬇️

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[ml-evs]

LGTM, thanks for spotting this

[ml-evs]

Actually, why don't the tests I just added fail, e.g.

optimade-python-tools/tests/models/test_structures.py

Lines 83 to 85 in d48b7a8

	{"chemical_formula_hill": "GeHSi"},
	"Elements in 'chemical_formula_hill' must appear in Hill order: ['H', 'Ge', 'Si'] not ['Ge', 'H', 'Si']",
	),

[ml-evs]

Actually, why don't the tests I just added fail, e.g.

optimade-python-tools/tests/models/test_structures.py

Lines 83 to 85 in d48b7a8

	{"chemical_formula_hill": "GeHSi"},
	"Elements in 'chemical_formula_hill' must appear in Hill order: ['H', 'Ge', 'Si'] not ['Ge', 'H', 'Si']",
	),

Okay mb, I see that test was updated (it just looked different to my local version)

[CasperWA]

Actually, why don't the tests I just added fail, e.g.

optimade-python-tools/tests/models/test_structures.py

Lines 83 to 85 in d48b7a8

	{"chemical_formula_hill": "GeHSi"},
	"Elements in 'chemical_formula_hill' must appear in Hill order: ['H', 'Ge', 'Si'] not ['Ge', 'H', 'Si']",
	),

Because I updated this exact test (as mentioned in the OP and found in the git diff).