xarray.dot is now based on da.einsum

[crusaderky]

Closes #2074
Requires dask >= 0.17.3

[shoyer]

Can we explicitly raise an informative error (maybe NotImplementedError) if any inputs are wrapping dask arrays and the version of dask is too old? Or maybe we could put that in duck_array_ops.einsum?

[crusaderky]

Done

E               AttributeError: module 'dask.array' has no attribute 'einsum': requires dask >=0.17.3

[shoyer]

It looks like the dot tests are now failing on test_dot because they don't find the right version of dask -- which makes sense because dask hasn't made the 0.17.3 release yet.

Let's wait until that happens (hopefully very soon: dask/dask#3452) before merging this. But we should also add a conditional version check for test_dot with dask. Something like:

        if LooseVersion(dask.__version__) < LooseVersion('0.17.3'):
            pytest.skip("needs dask.array.einsum")

(The appveyor failures should be fixed if you merge in master.)

[crusaderky]

@shoyer integrated your suggestion

[shoyer]

thanks @crusaderky !