Mark plot-based genetic tests as manual

[SrijaVuppula]

Summary

This PR marks plot-producing genetic tests as manual so they are separated from the regular automated test runs.

Changes

• add @pytest.mark.manual to test_single_point_crossover
• add @pytest.mark.manual to test_speed_crossover
• add @pytest.mark.manual to test_weight_determination_for_solution_selection

Verification

Ran:

./.venv/bin/pytest -m "manual" tests/test_genetic.py tests/test_genetic_mcdm.py -q
# Result:
- 7 passed
- 1 skipped
- 13 deselected

Ran:

./.venv/bin/pytest -m "not manual" tests/test_genetic.py tests/test_genetic_mcdm.py -q
# Result:
- 13 passed
- 8 deselected

Notes

test_single_point_crossover remains skipped with the existing skip reason:
Test needs modified route array.