Add tests
Add PGM examples but i'm not sure it gives correct answers yet.

Need a unit test for the pgm stuff
Flattening nested distributions with mapcat
Fixed the tail recursion issue
Representing distributions for Bayesian networks as hash-maps
Updated sampling given a predicate. Added some more stuff for the grass network.
Fixed probability function
changed true/false predicate and updated examples
Probability function corrected and debug keyword added. Examples updated.