Heat exchanging: radiator, icing & better optimization

[ngomezve]

This PR introduces models of radiators to reject residual heat, which would be required by electric systems with batteries or fuel cells. It reuses the existing heat exchanger models and adds new functions for the design and analysis of radiators.

In addition, the following changes are made:

• The heat exchanger optimization now uses the COBYLA algorithm instead of Nelder-Mead with penalty functions.
• The minimum heat exchanger temperature at every design point is stored to check for icing.

[codecov]

Codecov Report

Attention: Patch coverage is 96.44970% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.62%. Comparing base (430cfd8) to head (b8dcf1f).
Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
src/engine/hxfun.jl	96.44%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   73.41%   73.62%   +0.21%     
==========================================
  Files          77       80       +3     
  Lines       13908    13598     -310     
==========================================
- Hits        10211    10012     -199     
+ Misses       3697     3586     -111     

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