Fix oxidation doc composition

[shyamd]

This fixes the serialization of composition in oxidation doc:

• ensures StructureMetadata uses a charge-free composition or it won't bson into MongoDB
• ensures OxidationDocBuilder jsanitizes the doc before inserting to ensure proper conversion.

[codecov-commenter]

Codecov Report

Merging #217 (2cf79b2) into main (9638766) will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   85.36%   85.98%   +0.61%     
==========================================
  Files          36       37       +1     
  Lines        1777     1798      +21     
==========================================
+ Hits         1517     1546      +29     
+ Misses        260      252       -8     

Impacted Files	Coverage Δ
...lders/emmet/builders/materials/oxidation_states.py	100.00% <100.00%> (ø)
emmet-core/emmet/core/structure.py	100.00% <100.00%> (ø)
emmet-core/emmet/core/oxidation_states.py	92.68% <0.00%> (+19.51%)	⬆️

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