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)

Agent.__init__ now takes a program parameter. A few more nonlocal-state-variable problems found and fixed. One agent program rewritten as a callable instance since it uses both state variables and abstract methods.

Moved agent-program definition back into the constructors to get rid of all the copying between parameters, instance vars, and nonlocals. Fixed nonlocal state-var bug in ReflexAgentWithState.