Replaced Learner with a functional interface. This is a pretty big change you might especially want to review.

Fixed DataSet doc comments. Added rep-invariant check. DecisionTreeLearner now breaks ties on leaves randomly. NaiveBayesLearner now collects stats only on input fields. Minor cleanup.

Lined up DataSet params with the documented fields. Documented extra param. Fixed .sanitize() to replace with None (the old code no longer did quite what was documented).

Patched bug: enumeration_joint_ask gave wrong answers when the query variable overlaps the evidence.

Fixed: enumerate_joint_ask would raise KeyError on a combo of var values that didn't happen to be set. Though maybe this is too lenient, catching KeyError in ProbDist.__getitem__? (instead of in JointProbDist)