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
I must admit, I'm a bit out of the loop on the various changes to the Lexicons: Kate's changes, Adam's fixes and alternative method, and now these changes.  Emma and Kate:  Could you look at these changes in detail and comment?