Turn opacity-scale into a multimethod that dispatches on mode

[zane]

Currently we switch between opacity scales like so:

(let [make-opacity-scale (case mode
                           :resampling resampling/make-opacity-scale
                           :prior prior/make-opacity-scale)
      opacity-scale (make-opacity-scale (map :log-score curves))])

Instead we should define a multimethod curve-fitting.core/opacity-scale (or maybe curve-fitting.sketches/opacity-scale that takes two arguments: The mode and the log scores.