MAINT: Fix a documentation failure and deprecate usage of Molecule.get_formula

[BvB93]

No description provided.

[codecov]

Codecov Report

Merging #108 (cfb0b1f) into master (cbad6ff) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   41.62%   41.64%   +0.02%     
==========================================
  Files          35       35              
  Lines        2777     2778       +1     
==========================================
+ Hits         1156     1157       +1     
  Misses       1621     1621              

Impacted Files	Coverage Δ
nanoCAT/__version__.py	100.00% <100.00%> (ø)
nanoCAT/recipes/__init__.py	100.00% <100.00%> (ø)
nanoCAT/recipes/coordination_number.py	100.00% <100.00%> (ø)

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