Read a mol repr from an Arkane YAML file if given

[alongd]

Save a mol representation in an Arkane YAML file under a mol key and read the mol object properly if given.

A twin PR of https://github.com/ReactionMechanismGenerator/RMG-Py/pull/2447

[codecov[bot]]

Codecov Report

Merging #663 (5089842) into main (02a83e7) will increase coverage by 0.02%. The diff coverage is 71.79%.

@@            Coverage Diff             @@
##             main     #663      +/-   ##
==========================================
+ Coverage   73.19%   73.21%   +0.02%     
==========================================
  Files          99       99              
  Lines       26304    26359      +55     
  Branches     5502     5513      +11     
==========================================
+ Hits        19253    19299      +46     
- Misses       5688     5692       +4     
- Partials     1363     1368       +5     

Flag	Coverage Δ
unittests	73.21% <71.79%> (+0.02%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
arc/processor.py	16.21% <0.00%> (-0.09%)	:arrow_down:
arc/parser.py	83.82% <33.33%> (-0.22%)	:arrow_down:
arc/species/species.py	71.18% <53.12%> (-0.14%)	:arrow_down:
arc/plotter.py	27.77% <83.33%> (+0.38%)	:arrow_up:
arc/plotter_test.py	98.85% <94.44%> (-1.15%)	:arrow_down:
arc/common.py	83.26% <100.00%> (+0.38%)	:arrow_up:
arc/reaction_test.py	89.44% <100.00%> (+0.06%)	:arrow_up:
arc/species/species_test.py	99.80% <100.00%> (+<0.01%)	:arrow_up:

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