MAINT: Backport the PLAMS <= 1.5.1 Molecule.get_formula method

[BvB93]

Xref #209

Backport in preparation for the changes introduced in SCM-NV/PLAMS@6100836,
which switches Hill-system-based formula notation rather than the previous purelly alphabetical notation.

[codecov]

Codecov Report

Merging #212 (72d90c1) into master (23a3466) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
+ Coverage   67.43%   67.46%   +0.03%     
==========================================
  Files          48       48              
  Lines        3900     3904       +4     
  Branches      699      701       +2     
==========================================
+ Hits         2630     2634       +4     
  Misses       1114     1114              
  Partials      156      156              

Impacted Files	Coverage Δ
CAT/attachment/ligand_anchoring.py	89.74% <100.00%> (ø)
CAT/data_handling/mol_import.py	73.22% <100.00%> (+0.11%)	⬆️
CAT/multi_ligand.py	77.77% <100.00%> (ø)
CAT/utils.py	70.43% <100.00%> (+0.39%)	⬆️

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