[WIP] Add much-needed tests

[mattmcdermott]

The reaction-network test suite is extremely lacking, at the moment. In order for this package to continue in a healthy way, I am devoting some time to writing tests for the various modules (using pytest).

Summary

• Added pytest files for each module
• Cleaned up docstrings throughout code
• Wrote tests for most modules (still need to do firetasks, fireworks, network)
• Added better docs

[codecov-commenter]

Codecov Report

Merging #45 (c721e5d) into main (315fcc4) will increase coverage by 33.36%.
The diff coverage is 86.33%.

@@             Coverage Diff             @@
##             main      #45       +/-   ##
===========================================
+ Coverage   21.81%   55.18%   +33.36%     
===========================================
  Files          44       47        +3     
  Lines        2168     1928      -240     
===========================================
+ Hits          473     1064      +591     
+ Misses       1695      864      -831     

Impacted Files	Coverage Δ
src/rxn_network/core/__init__.py	100.00% <ø> (ø)
src/rxn_network/core/calculator.py	100.00% <ø> (ø)
src/rxn_network/core/cost_function.py	100.00% <ø> (ø)
src/rxn_network/core/solver.py	54.76% <ø> (ø)
src/rxn_network/data/__init__.py	100.00% <ø> (ø)
src/rxn_network/firetasks/build_inputs.py	0.00% <0.00%> (ø)
src/rxn_network/firetasks/parse_outputs.py	0.00% <ø> (ø)
src/rxn_network/firetasks/run_calc.py	0.00% <0.00%> (ø)
src/rxn_network/firetasks/utils.py	0.00% <ø> (ø)
src/rxn_network/fireworks/__init__.py	0.00% <0.00%> (ø)
... and 57 more

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