Feature request: Adding missing subsets and subsets_members implementation to Ubergraph implementation

Is it possible to add [subsets()](https://incatools.github.io/ontology-access-kit/packages/interfaces/basic.html#oaklib.interfaces.basic_ontology_interface.BasicOntologyInterface.subsets) and [subset_members()](https://incatools.github.io/ontology-access-kit/packages/interfaces/basic.html#oaklib.interfaces.basic_ontology_interface.BasicOntologyInterface.subset_members) methods to Ubergraph implementation?

Start with the linked BasicOntologyInterface documentation for subsets() and subset_members(), then locate the Ubergraph implementation in the repository. Compare how analogous implementations provide these methods and identify the relevant tests or entry points. Done means Ubergraph supports both methods with behavior matching the interface contract.

Written by the indexing model from the issue text.