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 :warning:
src/cryo_tank/tankWmech.jl	0.00%	2 Missing :warning:
src/cryo_tank/tankWthermal.jl	66.66%	2 Missing :warning:
src/cryo_tank/tanktools.jl	90.90%	2 Missing :warning:
src/cryo_tank/pressure.jl	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.