Improvements to heat exchanger models

[ngomezve]

This PR introduces minor changes to the heat exchanger models:

• The change in properties across the thermal boundary layer is now modeled via a wall temperature.
• The tube wall thickness is now calculated for a design pressure difference; for the turbofan cases, this is the maximum pt3.
• Rather than replacing the effectiveness by 95% of the maximum possible one, the code returns an error if the effectiveness is above the maximum one (to help with optimization).
• Added function to reset previous enthalpy and pressure changes across HX at the start of wsize, to prevent previous aircraft HX parameters from leaking into a new aircraft.
• Fixed issue in struct initialization, wherein the initial struct values may be taken from existing memory.
• Replaced hardcoded material parameters with new StructuralAlloys
• New tests for the thermal and pressure drop models. New tests for the HX design and operation function.

In addition, the structural alloy database has been modified to also include thermal conductivity.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.25490% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 76.41%. Comparing base (c224dd9) to head (78ba4b2). Report is 36 commits behind head on main.

Files	Patch %	Lines
src/engine/hxfun.jl	87.23%	12 Missing :warning:
src/engine/tfoper.jl	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #46 +/- ## ========================================== + Coverage 74.73% 76.41% +1.67% ========================================== Files 69 69 Lines 13089 13169 +80 ========================================== + Hits 9782 10063 +281 + Misses 3307 3106 -201 ```

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