Refactoring: ContinuousNode introduced, moved stuff from Discrete and Continuous Node so generic Node, created MCMC example for multiple continuous Models

Contstructing a MarkovChain with MetropolisHastings is now working for Networks which consist only of LinearGaussian nodes or ProbabilityTables. Additionally it is now possible to specify the form of densityfunction that one expects for PosteriorMarginal. Learning a Gaussian from a MarkovChain is also possible