Advances in heat exchanger models

[ngomezve]

This PR introduces minor improvements to the models for heat exchangers:
-The fuel can now have a non-zero enthalpy of vaporization. If there is no heat exchanger, this is accounted for in the burner, where the fuel is vaporized. With heat exchangers, the fuel gets vaporized before entering the burner.
-The heat exchanger mass and diameter are now stored, so that constraints can be applied to them.
-The heat exchanger additional mass fraction is now an input.
-The additional shaft length required due to the heat exchangers is now modeled.
-A relaxation factor is added to the heat exchanger properties in wsize, so that it does not change as much across iterations and potentially speeds up convergence.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.20290% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.36%. Comparing base (b579c6a) to head (97b42f6).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
src/engine/hxfun.jl	90.90%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   71.89%   73.36%   +1.46%     
==========================================
  Files          77       77              
  Lines       13715    13878     +163     
==========================================
+ Hits         9861    10182     +321     
+ Misses       3854     3696     -158     

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