Add an arithmetic_compat option to xr.set_options

[mjwillson]

This determines how non-index coordinates of the same name are compared for potential conflicts when performing binary operations.

The default is arithmetic_compat='minimal', which matches the previous behaviour.

I had originally planned to set this up to migrate the default to arithmetic_compat='override' alongside the wider migration of default to `compat='override' elsewhere. However I've held off on this for now pending a decision on what to do anything about the original motivation of compat='minimal' here which was to drop clashing scalar coordinates that arise from things like arr[0] + arr[1].

I think this is nice to have in general, complimenting the arithmetic_join setting and making sure we have the same degree of control over this for arithmetic operations as we do for things like xr.merge. But the immediate motivation was #10924 where we need to skip equality checks on coordinates in situations where we want to JIT a function that uses jax arrays for coordinates.

• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

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[mjwillson]

Thanks, have now added a DataTree test too.