Add context for callback to be removed from sim signal

[DominicOram]

When writing tests I want to simulate some basic signal behaviour e.g. writing to one signal will put to another. However, I would like to make sure this behaviour is properly cleaned up between tests. I can simulate with set_sim_callback and set_sim_value but there is no way to remove this callback.

Acceptance Criteria

• There is a context that removes callbacks from simulated signals on cleanup

[coretl]

I like this being a context, then making it fire on put rather than on every update, as that is what the tests want

[DominicOram]

Taken myself off this as @coretl said they will find someone else to pick it up. It's not urgent from our perspective

[coretl]

I think this will be fixed if we have a generic Mock backend in #94

[coretl]

Fixed by #94