Fixed thrust calculation when engine is choked

[ngomezve]

This PR addresses issue #95, in which the thrust calculation is incorrect if the engine is choked. The code currently assumes that the far wake is isentropically expanded to atmospheric pressure, which would require a variable area convergent-divergent nozzle. Most aircraft turbofans have convergent nozzles only, so the thrust calculation is slightly different when there is choking (back pressure needs to be accounted for).

The PR fixes the thrust calculation while keeping all outputs and station notations. For the default sized case, the thrust can change by up to 1.3% after making these changes. Stations 6 and 8 were only used to calculate the thrust and could be eliminated after this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 42 lines in your changes missing coverage. Please review.

Project coverage is 4.80%. Comparing base (430cfd8) to head (0e10f38).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
src/engine/tfoper.jl	0.00%	29 Missing ⚠️
src/engine/tfsize.jl	0.00%	13 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (430cfd8) and HEAD (0e10f38). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (430cfd8)	HEAD (0e10f38)
	3	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #96       +/-   ##
==========================================
- Coverage   73.41%   4.80%   -68.62%     
==========================================
  Files          77      76        -1     
  Lines       13908   13447      -461     
==========================================
- Hits        10211     646     -9565     
- Misses       3697   12801     +9104     

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