fix: selection with zarr arrays

[jhamman]

This is a partial solution to #2133. It works but does so in a potential suboptimal way -- materializing the entire indexer before indexing. A deeper solution would include streaming indexer chunks from the selector to oindex/vindex but I think that can wait until later.

closes #2133

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[jhamman]

@dstansby and @normanrz - I believe we want something like I've written here but am not clear if we can do this yet. Any ideas?

[dstansby]

I don't see a technical reason why it isn't possible to make Array a generic type over data types.

[jhamman]

happy to do this, can you point me in the right direction?