Moving alarm network data into a subfolder
Adding pydot (Python 3 compatible)
Refactoring nodes so that each has a parents property
Network sampler now skips over nodes for which is_pruned = True

Also minor tweaks to make code more readable.
Modified Bernoulli node to only warn if number of entries in probability table doesn't fit number of parents

so that we don't have to provide probability tables when testing pruning.
Versioning initial pruning code
Schachter paper on Bayes-Ball pruning algorithm