Get xarray.testing.assert_identical to work on datasets containing ManifestArrays

[TomNicholas]

We should be able to get xarray.testing.assert_identical to work on datasets containing ManifestArrays. I'm not sure why it doesn't already work (at least for the case of no indexes).

AssertionError: Left and right Dataset objects are not identical

Differing coordinates:
L   ocean_time  (ocean_time) float64 16B ManifestArray<shape=(2,), dtype=floa...
R   ocean_time  (ocean_time) float64 16B ManifestArray<shape=(2,), dtype=floa...
L   s_rho       (s_rho) float64 240B ManifestArray<shape=(30,), dtype=float64...
R   s_rho       (s_rho) float64 240B ManifestArray<shape=(30,), dtype=float64...
Differing data variables:
L   lat_rho     (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
R   lat_rho     (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
L   Cs_r        (s_rho) float64 240B ManifestArray<shape=(30,), dtype=float64...
R   Cs_r        (s_rho) float64 240B ManifestArray<shape=(30,), dtype=float64...
L   h           (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
R   h           (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
L   lon_rho     (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
R   lon_rho     (eta_rho, xi_rho) float64 567kB ManifestArray<shape=(191, 371...
L   hc          float64 8B ManifestArray<shape=(), dtype=float64, chunks=()>
R   hc          float64 8B ManifestArray<shape=(), dtype=float64, chunks=()>
L   salt        (ocean_time, s_rho, eta_rho, xi_rho) float32 17MB ManifestArr...
R   salt        (ocean_time, s_rho, eta_rho, xi_rho) float32 17MB ManifestArr...
L   zeta        (ocean_time, eta_rho, xi_rho) float32 567kB ManifestArray<sha...
R   zeta        (ocean_time, eta_rho, xi_rho) float32 567kB ManifestArray<sha...

Originally posted by @scottyhq in #143 (comment)