Fix OptimalPropeller.py for a hub smaller than the first station

[ManasBagul23]

Proposed Changes

i_hub in SU2_PY/OptimalPropeller.py was only assigned when a radial station lies inside the hub, so a hub radius smaller than R/stations (e.g. no hub) stopped the script with NameError. It now starts at the first station.

Checked with 50 stations, CT 0.1, R 1.0, J 0.5, Vinf 10: with r_hub = 0 the script now runs (dCT integral 0.1000, dCP integral 0.0618), and with r_hub = 0.1 the generated ActuatorDisk.dat and ActuatorDisk.cfg are identical to develop.

Related Work

Closes #2918.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.