DataArray.argsort should be deleted

Originally posted to https://groups.google.com/forum/#!topic/xarray/wsxeiIPLhgM

DataArray.argsort() appears to simply wrap the result of DataArray.values.argsort() in a same-shape DataArray. This is semantically nonsensical. If anything the index on the resulting argsort() values should simply be range(len(da)), but that adds little to the underlying numpy structure. And there's not much reason to have da.argsort() simply return the (raw) result of da.values.argsort(). So really DataArray.argsort() should simply be deleted.

On the other hand a new function DataArray.rank() that wraps da.values.argsort().argsort() (note repeated call to ndarray.argsort()) in the structure of the original DataArray would make sense, and perhaps even be useful... (Note that I'm not claiming that .argsort().argsort() is the fastest way to calculate this, but it's probably good enough, at least for an initial implementation.)

Start by locating the DataArray.argsort implementation and its tests, then read the issue discussion linked from the original report. Confirm whether the requested removal is still appropriate and whether the proposed DataArray.rank addition is in scope; done means the agreed API behavior is reflected in the implementation and tests.