#194 Fix in-between state of half-initialized propagator primitives

[Ulf Lorenz]

What and Why

The propagator primitives are currently set up in two steps. First they are created, then they are usually assigned to a Propagator instance, where they also get their timestep from. In-between these two steps, they are in some half-setup limbo where they cannot be used correctly. This should be fixed.

• Suggestion: Make a factory for each propagator primitive; the normal arguments set up the factory, when you also give the timestep, you get a primitive.
• Suggestion: Move the timesteps to the primitives.

Priority is rather low, but since this is a code cleanup issue that does change the interface, I put it into the 0.4 target.

A similar issue occurs in other contexts, e.g., DVROperator derived classes, grid classes. Are there also patterns to simplify these?

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I chose the simpler approach presented here where the timestep becomes an argument of the solver constructors. The DVROperator issue has very low priority because it is not user-visible and has been deferred to [#251].