Add module-level R_GAS constant value to diffusion.py

[RDaxini]

Describe your changes

Replace hard-coded magic number 0.0083144626 with a module-level R_GAS (gas constant) value

Issue ticket number and link

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist before requesting a review

• I have performed a self-review of my code
• Code changes are covered by tests.
• Code changes have been evaluated for compatibility/integration with Scenario analysis (for future PRs)
• Code changes have been evaluated for compatibility/integration with geospatial autotemplating (for future PRs)
• New functions added to init.py
• API.rst is up to date, along with other sphinx docs pages
• Example notebooks are rerun and differences in results scrutinized
• What's new changelog has been updated in the docs

[codecov-commenter]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.72%. Comparing base (51172fd) to head (5b76b1a).
⚠️ Report is 459 commits behind head on development.

Files with missing lines	Patch %	Lines
pvdeg/diffusion.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #313      +/-   ##
===============================================
+ Coverage        71.66%   74.72%   +3.06%     
===============================================
  Files               40       41       +1     
  Lines             4524     4823     +299     
===============================================
+ Hits              3242     3604     +362     
+ Misses            1282     1219      -63     

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[martin-springer]

lgtm! thanks for cleaning this up!