Moving kmeans from cuml to Raft

[lowener]

This PR replace the current KMeans of Raft with cuml's implementation.
Closes #28.
It is using the new device_*_view for the API.

[cjnolet]

Implementation is looking great so far. Most of my comments are related to small API things.

[cjnolet]

@lowener as part of this PR process, I'd like for cuml to be using the raft version so that we don't end up with two different versions for a short time. This happened with the class labels primitives on cuml and they've since diverged, unfortunately. I'm a tad less concerned about this one if you are planning to refactor the cuml side next.

I also understand that cugraph isn't using the new formal kmeans API but as this is header-only we have seen some unexpected surprises pop up for various reasons in the past and I'd like to avoid breaking the builds for either project, even if only for a short period.

[ajschmidt8]

Removing ops-codeowners from the required reviews since it doesn't seem there are any file changes that we're responsible for. Feel free to add us back if necessary.

[cjnolet]

rerun tests

[cjnolet]

@gpucibot merge