Implemented enumeration_ask algorithm, Fig. 14.9.

Added methods to the BayesNet class to needed for
enumeration_ask: variable_node, variables, variable_values.

Added Fig. 14.11a sprinkler network.

Completed/corrected prior_sample.

Implemented rejection_sampling.

The ProbDist normalize method now uses floating point division.

Implemented likelihood_weighting.

ProbDist constructor has an optional freqs (frequencies) argument.

Added function condsamp (samples from a CPT).

Additional doctest cases.