dask: Data.unique

[davidhassell]

No description provided.

[sadielbartholomew]

All good and ready to merge, with a nice extension to the unit test, though if I was to be harsh I might suggest first including also in the unit test a small block to cover the various data type possibilities ("fibUS" as set here) and in particular the case of a string-type data array, an example I checked manually in reviewing, since really we are only checking integer arrays at present?).

[davidhassell]

Harsh is fine! Data type unit tests: ae13c20

[sadielbartholomew]

Bonus points for adding the new commit, which I have sanity checked and has added my suggestion in an elegant way. All good to merge.

[davidhassell]

For future reference - the da.unique function was not used in this PR, we used da.reduction instead, because the former does not deal very well with missing data. I had a look at modifying da.unique to cope, but a) it was not trivial; b) cf.Data doesn't need (yet) the return_* keywords; and c) the dask folk might change it to be a reduction at some stage (dask/dask#2851). For these three reasons, it was preferable at this time to implement a missing-data-friendly "unique", without the return_* keywords as a standard cf.data.Collapse method.