Implement OPTICS

[RenatoGeh]

OPTICS (Ordering Points To Identify the Clustering Structure) is a clustering algorithm similar to DBSCAN. DBSCAN's major weakness is density tuning. OPTICS attempts to address this issue by ordering points and choosing the best epsilon.

We currently have an incomplete OPTICS implementation at utils/cluster/optics.go. LearnSPN relies heavily on both clustering and variable independence, and having OPTICS should increase its performance.

This isn't a priority though, as plenty other more interesting structure learning algorithms have sprung up recently.