Use __getstate__ and __setstate__ for propagator serialization

[akoumjian]

Rather than instantiate propagators in the workers implicitly by using self.dict and self.class, the proper way to do this is to take advantage of dunder methods getstate and setstate. Now we can pass in propagator instances directly without fear of nasty stateful bits or non-initialization keyword arguments getting passed around.

Here we implement them as required in the ABC. Tests will fail until pointing to updated adam-assist in the development requirements.