Deprecate indexing with non-aligned DataArray objects

[shoyer]

Currently, we strip labels from DataArray arguments to [] / .loc[] / .sel()/ .isel(). But this will break when we finally add indexing with alignment (#974).

We could start raising deprecation warnings now so users can stop relying on this functionality that will change.

[jhamman]

@shoyer / @fujiisoup - can this be closed now?

[shoyer]

Yes, we decided not to do this.