Added stream activate and deactivate methods

[philippjfr]

Adds Stream.activate and Stream.deactivate methods to streams to be triggered just before updating parameters and just after triggering the stream respectively. This allows defining custom actions after an event, e.g. in order to reset the value after an event has been processed.

[jlstevens]

I think it is a bit odd that activate is called only if you use update. This means:

• If you use update with trigger=False, then activate is called but deactivate isn't.
• If you use the trigger classmethod directly then deactivate is called by activate isn't.

Can both activate and deactivate be used in the trigger classmethod?

[jlstevens]

Looks good. Merging.