Distinction between max payload and max pax

[ngomezve]

This PR is a revived form of PR #63. It introduces the exit_limit as a way to specify the maximum number of seats in the cabin. This is stored as a field in fuselage.cabin. In addition, the name of the input max_pax is changed to the more accurate max_payload_in_pax_equivalent.

The code now uses the exit limit to resize the fuselage where appropriate. The PR also fixes a very minor bug in the double-decker case (a parg parameter had been left behind).

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 33.33333% with 16 lines in your changes missing coverage. Please review.

Project coverage is 73.31%. Comparing base (018c044) to head (1c5076d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/structures/size_cabin.jl	0.00%	9 Missing ⚠️
src/structures/update_fuse.jl	22.22%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
- Coverage   73.32%   73.31%   -0.01%     
==========================================
  Files          77       77              
  Lines       13846    13851       +5     
==========================================
+ Hits        10152    10155       +3     
- Misses       3694     3696       +2     

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