Fix STATION function names in SU2_PY: SU2_GEO numbers stations from 1

[nikita-ageev]

SU2_GEO writes STATION1_* ... STATIONn_* for the stations in GEO_LOCATION_STATIONS, while optnames_geo listed STATION0_* ... STATION19_. STATION20_ was therefore rejected by SU2.eval.func/grad as an unknown function name, and STATION0_* is never written.

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Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.

PR Checklist

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• 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. — Not necessary: no numerical change; existing station constraints (STATION1_* ... STATION5_) in shape_opt_euler_py are unaffected, and the reproduction above covers STATION20_.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary. — config_template.cfg: documented the 1-based STATION numbering (1 to 20).