Update math lower bound capacity setting

[brynpickering]

Fixes #643

Hopefully this does the trick @sstroemer!

Summary of changes in this pull request

• Moved capacity variable lower bound setting to own constraints.
• Added equivalent constraints for area_use and source_cap.

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (959fea7) to head (8352f20).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #700   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          29       29           
  Lines        4060     4060           
  Branches      580      580           
=======================================
  Hits         3897     3897           
  Misses         72       72           
  Partials       91       91