Fixed several bugs related to evidence. Hopefully included LinearBeta, still needs testing though. Changed sample_global and sample_local once again in ContinuousNode

examples/MCMC_Continuous.py läuft mit exponentialverteilung->linearGauss, Evidenz als Klassen und Kontinuierliche PoE läuft

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