Force link between flow cap and purchased units in base math

[brynpickering]

Equivalent fix to the issue raised in #565.

Summary of changes in this pull request:

• Link purchased_units with flow_cap even when flow_cap_max has not been set by the user. Uses bigM to achieve this.

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 ✅

Comparison is base (36466f9) 95.76% compared to head (b8ed034) 95.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #566   +/-   ##
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  Coverage   95.76%   95.76%           
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  Files          24       24           
  Lines        3566     3566           
  Branches      777      711   -66     
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  Hits         3415     3415           
  Misses         86       86           
  Partials       65       65