Fix crash in fortran layer

[eytanadler]

Purpose

With particularly bad airfoil shapes, the IBLPAN function fails. Upon failure, it calls the STOP command. This kills not only the XFOIL instance, but also the whole Python script (not good during optimization...).

This PR removes the STOP command, instead setting fail flags accordingly and cleanly exiting back to the Python layer.

I also uncommented the residual printout in the real version of CMPLXFOIL so that the user gets some feedback on convergence behavior during the primal solve.

Expected time until merged

Now!

Type of change

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Documentation update
• [x] Maintenance update
• [ ] Other (please describe)

Testing

Checklist

• [ ] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.19%. Comparing base (7b16ff9) to head (b4070f8).

Files	Patch %	Lines
cmplxfoil/__init__.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ======================================= Coverage 66.19% 66.19% ======================================= Files 4 4 Lines 565 565 ======================================= Hits 374 374 Misses 191 191 ```

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[A-CGray]

Is it at all possible to add a test for this new behaviour? Or is it not possible to recreate a case where IBLPAN fails?

[eytanadler]

If it ever hits a STOP in Fortran, it will kill the program. That means that if the test ever fails, it won't be caught as a test failure and instead will kill the whole testing process. I can probably recreate a case, but if it fails it'll be very difficult to debug. Is that worth it? I think the fact that other regression tests still pass is a good sign that I haven't changed the underlying implementation.

On another note, do you have any idea what's wrong with readthedocs at first glance?