Adding coverage config file.

[astronomerritt]

Adding config file (.coveragerc) for coverage reports that can be used to omit certain files. Currently this excludes src/sorcha/utilities/citation_text.py.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does Sorcha run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa86227) 65.31% compared to head (efea425) 74.34%.
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #747      +/-   ##
==========================================
+ Coverage   65.31%   74.34%   +9.03%     
==========================================
  Files          67       66       -1     
  Lines        3451     3025     -426     
==========================================
- Hits         2254     2249       -5     
+ Misses       1197      776     -421     

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