Nearest Neighbors

I would like to see out of core nearest neighbor algorithms like those available in [`cuml`](https://docs.rapids.ai/api/cuml/stable/api/#id37) available in `dask-ml`.

I would hope this could be a matter of wrapping an existing out-of-core approximate neighbors library and handling an input `dask.array`.

Start by reviewing the cuML nearest-neighbor API linked in the issue and the dask.array interface mentioned there. Determine which out-of-core approximate-neighbors library could be wrapped and how a Dask array would be handled; done means a defined integration with working nearest-neighbor behavior and tests.

Written by the indexing model from the issue text.