feat: add function that converts chemical formula to hill notation

[yucongalicechen]

Task 1 in #333
@sbillinge ready for review! I splitted the issue into multiple steps, and this is the first one. For this issue I think this might be a better way to organize the substeps since everything is in one place.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a51b553) to head (ff9b26b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #335   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          516       519    +3     
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+ Hits           516       519    +3     

Files with missing lines	Coverage Δ
tests/test_tools.py	100.00% <100.00%> (ø)

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[sbillinge]

I am not sure this is the right approach. I think it might be more reusable if we take a diffpy.structure object and write a method that will output hill notation.