Add GasState modifiers

[jcurtis2]

Adds gas_state_scale and gas_state_molar_conc_to_ppb to allow the values of the GasState to be modified if needed - either based on a scaling factor and/or for unit conversion purposes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (1490a37) to head (406e604).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
+ Coverage   90.81%   90.85%   +0.04%     
==========================================
  Files          55       55              
  Lines        2569     2581      +12     
  Branches      144      144              
==========================================
+ Hits         2333     2345      +12     
  Misses        163      163              
  Partials       73       73              

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