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

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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