Convert kmeans backend to scikit-learn

[michael-petersen]

The old hand-written version of kmeans has probably outlived its lifespan, and should be converted to the implementation in scikit-learn, available here.

I think a reasonable plan would be to

• Implement a drop-in replacement for KMeans, which entails a bit of plumbing to get the attributes from scikit-learn mapped onto the exptool names
• Subsume KPlusPlus into Kmeans
• Update installation requirements to include scikit-learn

Anything else I've forgotten?