Address return value type for single-element access

[kgryte]

This PR seeks to address the issue discussed in gh-123 concerning single-element access by always returning an array, including when accessing a single element. This aligns with accelerator array libraries which always return arrays.

Resolved: the PR includes a TODO for the type hint for __getitem__, where key may be one of either int, <array>, slice, or selection tuple.

[rgommers]

The PR includes a TODO for the type hint for __getitem__, where key may be one of either int, <array>, slice, or selection tuple. Not clear to me how we want to communicate the type hint in this case.

How about Union[int, Tuple[int, ...], slice, <array>]?

[kgryte]

Updated this PR to include the type annotation for the index key.

[rgommers]

LGTM, thanks @kgryte