Re-implementation of collapsed gibbs sampler handlers + cleaning

* re-implementation of collapsed gibbs sampler without handlers, based on sampling from factors only
* removal of Categorical Factors, because they can all be represented by Discrete Factors using DiscreteVar as child and/or parent type
* resolves issue #178
* small tests were conducted for training LDA and it worked
* still a little bit slow compared to the efficient LDA implementation, but at least as fast as previous implementation of collapsed gibbs sampling
Minor note:  I subscribe to the FSF's "avoid ASCII art" philosophy.  I know it is common Javadoc and Scaladoc practice to use extra leading asterisks in multi-line comments, but for the future, my preference is to leave them out.  Less ink; less distracting.  

As for horizontal separators, where some people would use /********/ ASCII art lines, I prefer extra whitespace.