Discrete Distribution chooses a random symbol
Normal distribution generates a random value
IIDModel uses distribution to generate a random symbol/value
removing unused field from DiscreteDistribution
hiding createMember of NormalDistribution
Refactoring AlphabetSpec
refactoring DiscreteIIDModelSpec
refactoring ContinuousIIDModelSpec
refactoring DiscreteDistributionSpec
refactoring NormalDistributionSpec