RadiusNeighborsClassifier Support

[SebNav]

Hello, I like this project and regularly use several of the algorithms currently supported, such as SVC, KNN classifier, and DBSCAN. Nowadays, I am looking for some kind of optimized and fast implementation of RadiusNeighborsClassifier. It would be good to have that implementation in this library.
Thank you very much for your work.

[syakov-intel]

Currently we do not plan to implement this algorithm without enough evidence of its broad usage. Although contribution would be welcome!