Speed up the airfoil drag calculations

[askprash]

This PR makes some improvements to the airfoil drag calculations to speed up the airsection calculations by roughly ~5x. The airfoil type wasn't really type stable/ it's field types weren't inferable by the compiler. Also the spline evaluations have been cleaned up a little and optimized for better performance.

On my M2 mac the performance of the airfoil section calculations are:

julia> bench = @benchmarkable aerodynamics.airfun($clp, $toc, $Mperp, 
                                               $airfoil_section) seconds = 30 evals = 100
Benchmark(evals=100, seconds=30.0, samples=10000)

julia> bench_airfun = run(bench)
BenchmarkTools.Trial: 10000 samples with 100 evaluations.
 Range (min … max):  696.250 ns …  1.106 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     701.670 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   710.746 ns ± 24.497 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

  ▇█▇▆▅▅▄▂▂▂▁▁    ▃▄▃▃▂▁                                       ▂
  █████████████▇▇██████████▆▇▇▆▆▅▆▆▆▇▅▇▆▆▆▅▆▄▆▅▆▅▄▅▄▃▅▄▄▄▅▃▄▂▅ █
  696 ns        Histogram: log(frequency) by time       824 ns <

 Memory estimate: 96 bytes, allocs estimate: 4.

This PR:

julia> bench_airfun = run(bench)
BenchmarkTools.Trial: 10000 samples with 100 evaluations.
 Range (min … max):  131.250 ns … 392.500 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     133.750 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   134.427 ns ±   7.138 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

              ▂  ▆  █  ▆  ▄                                      
  ▂▁▁▃▁▁▃▁▁▆▁▁█▁▁█▁▁█▁▁█▁▁█▁▁█▁▁▅▁▁▄▁▁▃▁▁▂▁▁▂▁▁▂▁▁▂▁▁▂▁▁▂▁▁▂▁▁▂ ▃
  131 ns           Histogram: frequency by time          140 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.43%. Comparing base (8ee97c4) to head (383570a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   78.49%   78.43%   -0.06%     
==========================================
  Files          79       79              
  Lines       13499    13514      +15     
==========================================
+ Hits        10596    10600       +4     
- Misses       2903     2914      +11     

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[ngomezve]

The PR looks good to me. The aerodynamic functions in airfun.jl in general are the least readable in all of TASOPT: they have essentially no comments or documentation and the variable names are not descriptive. Since the PR is not any worse in this regard than main, I believe it can be pulled in.

[ngomezve]

This function in general is very poorly commented. Not relevant for this PR but this is very hard to interpret.

[ngomezve]

These functions have no comments or documentation.

[ngomezve]

It would be good to comment on why the integer division is used