Cryogenic fuels: hold times, more insulation options and tests

[ngomezve]

This PR addresses the models for aircraft with cryogenic fuels in insulated tanks:

• The effect of hold times at departure or arrival can now be modeled; this will affect fuel venting.
• Radiation to the fuselage has been eliminated from the thermal models; the effect was negligible and the model was sketchy.
• Added vacuum microspheres as an insulation option.
• Fixed a bug in the balance-field calculation with heat exchangers, wherein the effect of the heat exchanger was not being accounted for.
• Fixed a small bug in the tank time integration, which could ignore the takeoff time.
• Minor changes to gasfun.jl and the models in CryoTank to eliminate type promotion and improve speed.
• New test for hydrogen aircraft sizing.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.27451% with 14 lines in your changes missing coverage. Please review.

Project coverage is 78.09%. Comparing base (78ba4b2) to head (10f25f8).
Report is 5 commits behind head on main.

Files	Patch %	Lines
src/IO/save_model.jl	0.00%	7 Missing ⚠️
src/cryo_tank/tankWmech.jl	0.00%	2 Missing ⚠️
src/cryo_tank/tankWthermal.jl	66.66%	2 Missing ⚠️
src/cryo_tank/tanktools.jl	90.90%	2 Missing ⚠️
src/cryo_tank/pressure.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   76.41%   78.09%   +1.67%     
==========================================
  Files          69       71       +2     
  Lines       13169    13424     +255     
==========================================
+ Hits        10063    10483     +420     
+ Misses       3106     2941     -165     

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[askprash]

This is interesting, I wonder if insulation materials can be something that extends the Materials type in a future refactoring? E.g., something like ThermalInsulators like StructuralAlloys or something. This is fine for now though!