Adds more detailed descriptions to the mda8 variables.

[thorbjoernl]

Change Summary

Adds more detailed descriptions to mda8 variables which references the EU directive. New analyses that makes use of MDA8 calculation will have this new description display in the tool tip on aeroval.

Related issue number

https://github.com/metno/AeroToolsIssues/issues/69

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.97%. Comparing base (9e48af9) to head (8c6cd92).
Report is 38 commits behind head on main-dev.

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1320      +/-   ##
============================================
- Coverage     78.97%   78.97%   -0.01%     
============================================
  Files           136      136              
  Lines         20812    20815       +3     
============================================
+ Hits          16436    16438       +2     
- Misses         4376     4377       +1     

Flag	Coverage Δ
unittests	78.97% <ø> (-0.01%)	⬇️

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