Add options to reset generative sources

[sofian]

This is important to re-sync a Metro or an Osc object. We could also either add a reset() function, which would be the easiest. We could also choose to add similar functions as the Chronometer object, with start/stop/resume functions. But this would involve keeping track of whether the object has been started.

[sofian]

Question is: what word to use?

• reset() - would work but is currently used for resetting accumulators (eg. moving average, min max, etc.)
• start() - would make sense (especially if we implement the stop/resume/etc. function) but not so evocative (we have a similar problem with the Chronometer class)

[sofian]

Also: there would be an easy way to avoid using an extra boolean in the Metro and Osc classes by using the sign of one of the floats.

[sofian]

I think we should follow a common API similar to Chrono. See #92