Remnant issues from #3304; adding more type annotations

Remnant check boxes from #3304:

• the return value of find_redundant_units in remove_redundant.py. It currently returns a list of a two member list.
  • Should we update that to be a list of 2 member tuples? Is there a reason why it is current a list of lists?
• In sortingview_curation.py, there is # TODO @alessio you remove this after testing. I have not annotated this function since it would require importing BaseSorting since even in quotes, Pylance does not recognize it.

After ensuring most documented functions offer return values, I think there are instances where defining input type hints can be useful as the input type definitions are sometimes implicitly used in the return type annotation. For example, if we annotate CurationSorting to have "BaseSorting" as part of the input in def __init__(self, sorting: "BaseSorting", make_graph=False, properties_policy="keep"):, then the return values of .sorting() and .current_sorting() are automatically provided instead of Any. The importance of this may not be high as these are internal functions rather than external functions.

Review #3304 and inspect remove_redundant.py and sortingview_curation.py. Determine whether find_redundant_units should return tuples, and investigate the BaseSorting annotation and related input and return type hints. Done means the agreed annotations are added and the noted TODO is resolved without breaking type checking or tests.