Refactoring the fuselage sizing

[askprash]

@aditeyashukla starting a draft PR here so that others can also see what we've been up to and add any suggestions.

This PR refactors the fuselage sizing code to be more modular, easy to read and more efficient.

Some of the updates/features:

• Introduce the Fuselage, FuselageLayout, and a couple other types that make it easier to define and size the fuselage
• StructuralMembers are a generic type that can be used to represent various structural components - e.g., in the fuselage this may be the skin, the floor, etc.
• We introduce loads.jl that has lets us define Weights and more semantically calculate the moments of weights. Also have some basic coordinate system introduced (e.g., now we can associate loads/ weights with a Frame which defaults to a privileged WORLD frame. This can allow us to translate between frames more easily. We may potentially replace this with homogenous coordinates if we start to find it useful enough.

[askprash]

Fixed #68

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 58.49858% with 293 lines in your changes missing coverage. Please review.

Project coverage is 71.88%. Comparing base (78ba4b2) to head (cb36835). Report is 34 commits behind head on main.

Files	Patch %	Lines
src/IO/save_model.jl	0.00%	147 Missing :warning:
src/structures/loads.jl	54.79%	33 Missing :warning:
src/misc/fuselage_geometry.jl	66.66%	32 Missing :warning:
src/structures/update_fuse.jl	44.11%	19 Missing :warning:
src/misc/materials.jl	31.57%	13 Missing :warning:
src/IO/quicksave_load.jl	0.00%	10 Missing :warning:
src/sizing/wsize.jl	78.04%	9 Missing :warning:
src/IO/read_input.jl	87.69%	8 Missing :warning:
src/balance/balance.jl	75.86%	7 Missing :warning:
src/misc/layout.jl	84.61%	4 Missing :warning:
... and 6 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================== - Coverage 76.41% 71.88% -4.53% ========================================== Files 69 77 +8 Lines 13169 13684 +515 ========================================== - Hits 10063 9837 -226 - Misses 3106 3847 +741 ```

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