Part II of DF refactoring

Move more "chained" calculation steps into standalone objects/methods.

The only one missing is polarizability: It still requires a knowitall method on the DielectricFunctionCalculator. However it now works via a new Polarizability object so there's that. (Or should it be PolarizabilityData?)

Generally the DielectricFunctionCalculator now has a lot of _new_xxx() methods which return the new objects for each quantity we can calculate. It also has backwards compatible methods that return self._new_xxx(*args, **kwargs).unpack() which produces a tuple or whatever else that function originally returned.