Deprecate blind point parameter in Voronoi compuation

Summary

Mark the use_blind_points parameter in compute_individual_voronoi_polygons as officially deprecated, clean up all test usages, and ensure a DeprecationWarning is emitted when the parameter is passed.

Background / Context

use_blind_points has no effect on the computation and was kept for backwards compatibility. It is already documented as deprecated in the docstring, but the deprecation is not consistently enforced across the codebase — reference tests still pass it without expecting a warning.

Technical Details

• Parameter lives in pedpy/methods/method_utils.py — deprecation warning already emitted when the parameter is not None
• Reference tests in tests/reference_tests/reference_test.py still pass use_blind_points without pytest.warns
• Existing deprecation warning unit tests in tests/unit_tests/methods/test_compute_individual_voronoi_polygons.py

Acceptance Criteria

• Passing use_blind_points (any value) emits a DeprecationWarning
• A test verifies that passing the parameter yields a DeprecationWarning
• All tests specific to use_blind_points behavior are removed
• Reference tests no longer pass use_blind_points
• Reference data for verifying use_blind_points are removed

Start in pedpy/methods/method_utils.py, then inspect the existing deprecation tests in tests/unit_tests/methods/test_compute_individual_voronoi_polygons.py and the usages in tests/reference_tests/reference_test.py. Run the relevant tests first. Done means passing the parameter is covered by a DeprecationWarning test, obsolete behavior tests and reference data are removed, and reference tests no longer pass use_blind_points.