Partial equality checking

[TomNicholas]

WIP towards actually doing full element-wise equality checking between ManifestArray objects.

[dcherian]

Can this be avoided with sensible default options for concat? data_vars="minimal", coords="minimal", compat="override"?

[TomNicholas]

Can this be avoided with sensible default options for concat? data_vars="minimal", coords="minimal", compat="override"?

This __eq__ method isn't actually used during concat I don't think. I just implemented it so that I could call xrt.assert_equal on virtual datasets. This PR would make the result more useful in cases where some but not all of the chunks in two manifests are equivalent.