Support optimization of multiple points in batch spaces

[khurram-ghani]

Related issue(s)/PRs: None

Summary

Add support for batch optimization of multiple points per region/subspace in a TaggedMultiSearchSpace collection.

Fully backwards compatible: yes

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)

[hstojic]

I think it looks good, just a small comment for a test
it does look like we have a right test in place, checking that all optimised points are in the right box, that should be sufficient