Allow devices to save the current parameter state

Ok, I couldn't wait ...

For many reasons, it is very helpful to go back to a certain state. This change adds `save` and `restore` methods to `Parameter` and `Parameterizable` to save the current values on a stack, later to be restored.

We should still debate about the name though. Some possible alternatives:
- `take_snapshot`/`restore_snapshot`
- `push`/`pop`
- `stash`/`restore`
- etc.