Move some of the very old Pascal/special Gibbs sampling priors stuff that no one uses out of core. To enable this, change the loading of prior SequenceModel's for Gibbs sampling to loading by reflection (Dependency Injection...) rather than having them all mentioned in a big if/then/else. Mainly to enable this change, introduce a new ListeningSequenceModel for the conjunction of being a SequenceModel and a SequenceListener