partial cleaning of module directed; new CollapsedGibbsSamplerHandler

* new handler for collapsed gibbs sampler (for discrete values), which substitutes earlier special handlers
* removal of Categorical Factors, because they can all be represented by Discrete Factors using DiscreteVar as child and/or parent type
* resolves issue #178
* Introduces new traits that help during sampling: SeqGeneratingFactor, SeqAsParentFactor
* candidate selection for special cases can be done with new Mixin traits: (Plated)SelectedCandidatesCollapsedGibbsSamplerHandler
* small tests were conducted by training LDA a simple LDA model