Interface for restarts

[pabloferz]

This adds two ways of restarting runs:

1. From a stored Result (this is compatible with our MPI interface). One change introduced here is that the last snapshot of a simulation is now also part of Result instances. The interface looks like
   pysages.run(result: Result, context_generator: Callable, timesteps: int, *other_args, **kwargs)
   (that is, one would switch method by result when restarting).

2. From a SamplingContext (which is a renaming of our ContextWrapper). This is useful for interactive sessions, say in a jupyter notebook, or for debugging purposes.

FFS is left out for now as it is too different and requires other changes to its interface.

[trunk-io]

😎 Merged successfully (details).

[gustavor101]

The restart functionality works well with pickle files and multiple calls in notebooks, but a couple of examples could be useful for the user like showing that the loggers are now incorporated in SamplingContext and not in pysages.run as before.