Improved models for cryogenic fuel boiloff

[ngomezve]

This PR applies some minor improvements to the models for cryogenic fuels:

• Boiloff fuel is only vented when it would result in an increase in tank pressure (if the volume of the boiloff gas at saturation is greater than the volume that is made available by liquid boiloff and fuel burn).
• The mass of the ullage gas inside the tank is now accounted for.
• The latent heat of vaporization of hydrogen and methane is now a function of tank pressure.
• Added Mylar as a material option for the insulation, specifically for vapor barriers.
• Font size in tank diagrams for docs has been increased.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.63265% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 74.76%. Comparing base (c224dd9) to head (5b1a55b).

Files	Patch %	Lines
src/IO/read_input.jl	0.00%	4 Missing :warning:
src/structures/tankWthermal.jl	60.00%	4 Missing :warning:
src/structures/tankWmech.jl	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #29 +/- ## ========================================== + Coverage 74.73% 74.76% +0.03% ========================================== Files 69 69 Lines 13089 13111 +22 ========================================== + Hits 9782 9803 +21 - Misses 3307 3308 +1 ```

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[ngomezve]

I converted the PR back to draft to increase test coverage based on Codecov report.