Integrate Natural Breaks (Jenks)

So far `make-surface` allows for equal interval, quantile breaks, any hybrid of the previous two, or manual classification for vectorization. Integrate a natural breaks classifier using random raster samples (for speed).

cc: @heyitsgarrett @andreasviglakis

The issue names no files or tests. Start by tracing how make-surface currently selects equal-interval, quantile, hybrid, and manual classifications, then locate the raster-sampling entry point; done means a natural-breaks classifier using random raster samples is available for vectorization.

Written by the indexing model from the issue text.