Remove IndepVarComp from heat exchanger model so users can connect things to the DV values

[eytanadler]

Purpose

The heat exchanger model previously had an independent variable component connected to most inputs, such as channel geometry, number of channels in each dimension, etc. This prevented users from connecting outputs from other components to these inputs in the heat exchanger model. This fix removes the independent variable component and instead uses OpenMDAO's set_input_defaults. This means OpenMDAO will automatically connect an IndepVarComp with these values if nothing is connected by the user.

This change also allows the removal of the copy of all HXGroup's internals into the heat exchanger test script.

Type of change

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• Refactoring (no functional changes, no API changes)

Testing

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Checklist

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• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9dc5d69) 79.95% compared to head (3c7d7e1) 79.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   79.95%   79.95%   -0.01%     
==========================================
  Files          85       85              
  Lines        9301     9300       -1     
==========================================
- Hits         7437     7436       -1     
  Misses       1864     1864              

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[mariejvaucher]

Good job