Adding landing gear mass models

[ngomezve]

This PR introduces new landing gear mass models based on historical correlations. In the future, physics-based models could be included as well. The following changes are made:

• Two landing gear models are supported: the previous constant mass fractions and a model based on historical data, which incorporates dependencies on landing gear length and design.
• The landing gear parameters are stored in a new object in ac. The related parg parameters are eliminated.
• The balance() and mission!() functions now take in the ac object directly instead of the par arrays; these are then unpacked inside the functions. This makes the syntax more robust to changes in data storage.
• Documentation and tests for the landing gear models.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 73.08%. Comparing base (430cfd8) to head (10357e0).
Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
src/IO/read_input.jl	60.00%	8 Missing ⚠️
src/IO/save_model.jl	0.00%	7 Missing ⚠️
src/IO/outputs.jl	66.66%	2 Missing ⚠️
src/misc/aircraft.jl	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   73.41%   73.08%   -0.34%     
==========================================
  Files          77       82       +5     
  Lines       13908    13582     -326     
==========================================
- Hits        10211     9926     -285     
+ Misses       3697     3656      -41     

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