More intuitive interface to serialize objects to dict

[andrrizzi]

For now, the serialization system is based on calling directly __getstate__ and __setstate__. We should add to_dict and from_dict methods.

Note that many objects have __get/setstate__ methods that accept arguments that can be used to improve performance (e.g. SamplerState.__getstate__(ingore_velocities=False)) and should have an equivalent argument in to/from_dict.

See also #407.