Add a custom amount kites

[1-Bart-1]

• add option for custom amount of kites
• add a tutorial for creating just a tether (0 kites)
• Rename RamAirKite to SymbolicAWESystem and PointMassSystem to SystemStructure

[codecov]

Codecov Report

Attention: Patch coverage is 82.54364% with 70 lines in your changes missing coverage. Please review.

Project coverage is 85.99%. Comparing base (c4713e7) to head (81b22a5).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ram_air_kite.jl	76.88%	43 Missing ⚠️
src/mtk_model.jl	91.33%	11 Missing ⚠️
src/point_mass_system.jl	84.50%	11 Missing ⚠️
ext/KiteModelsControlPlotsExt.jl	75.00%	4 Missing ⚠️
src/precompile.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
+ Coverage   85.93%   85.99%   +0.06%     
==========================================
  Files           9        9              
  Lines        1856     1957     +101     
==========================================
+ Hits         1595     1683      +88     
- Misses        261      274      +13     

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