Spatial parameters cannot be time dependent if not solution-dependent

If we compute the tracer model in combination with a fluid that changes properties over time, we need the old properties to evaluate the storage.

The problem is, the volvars currently don't know anything about time, so we can't pass that information to the spatial params. That's why we currently assume that all secondary variables are either constant or solution-dependent but not time-dependent without being solution-dependent. This is a big drawback in general and a bug or at least an imprecision in the tracer model.