Refactor tilde/assume/observe interface

[phipsgabler]

Discussed here.

tilde_assume and tilde_observe currently fall back to different methods of tilde/dot_tilde, which are distinguished by their arguments. And these methods in turn call the assume and observe methods from the inference algorithm. Some inference algorithms also (or instead) overload methods of tilde directly.

Until now the tilde_{observe,assume} methods are only an internal wrapper and work backwards compatibly.

I propose

• assume, observe, and dot_{assume,observe} to be overloaded and used by inference algorithms, and
• tilde_{assume,observe} and dot_tilde_{assume,observe} for internal usage (unpacking stuff, and unifying the return values).