NamedArray tracking issue

@andersy005 I think it would be good to keep a running list of NamedArray tasks. I'll start with a rough sketch, please update/edit as you like.

• Refactor out NamedArray base class (#8075)
• publicize design doc: Scientific Python | Pangeo | NumPy Mailist
• Migrate VariableArithmetic to NamedArrayArithmetic (#8244)
• Migrate ExplicitlyIndexed array classes to array protocols
• MIgrate from *Indexer objects to .oindex and .vindex on ExplicitlyIndexed array classes
• https://github.com/pydata/xarray/pull/8870
• Migrate unary ops
• Migrate binary ops
• Migrate nanops.py
• Avoid "injecting" reduce methods potentially by using generate_reductions.py? (#8304)
• reprs and formatting.py
• parallelcompat.py
• pycompat.py (#8244)
• https://github.com/pydata/xarray/pull/8276
• have test_variable.py test both NamedArray and Variable
• Arrays with unknown shape #8291
• https://github.com/pydata/xarray/issues/8306
• https://github.com/pydata/xarray/issues/8310
• https://github.com/pydata/xarray/issues/8333
• Try to preserve imports from xarray.core/* by importing namedarray functionality into xarray.core/*

xref #3981

This is a broad tracking issue for NamedArray migration work, listing open areas such as unary and binary ops, nanops.py, formatting.py, pycompat.py, ExplicitlyIndexed array classes, and test_variable.py. Start by checking the linked issues and pull requests for current ownership and scope; done means completing one clearly scoped checklist item with its relevant tests updated.