Make get_unique_points more efficient

[uri-granta]

Related issue(s)/PRs: #781

Summary

Make get_unique_points more efficient and document its precise behaviour.

Fully backwards compatible: ish

The behaviour changes as we now use rounding to select points rather than a distance based loop. But the old behaviour was never fully documented, and the rounding approach should work fine for BatchTrustRegionBox.get_initialize_subspaces_mask which is the only place we currently use it.

PR checklist

• [ ] The quality checks are all passing
• [ ] The bug case / new feature is covered by tests
• [ ] Any new features are well-documented (in docstrings or notebooks)

[uri-granta]

Closing PR following offline discussion. Will create a separate PR for parallelising the existing greedy set cover algorithm.