Pacemaker features

[mhasself]

Two things I wanted to do with Pacemaker class, and couldn't:

• Use quantize with a period that was longer than 1s. E.g. freq=0.1 should have samples occur at timestamps that are 0 mod 10.
• Check & update the internal state, without sleeping. A check() function that returns False if it's not time to update; and returns True and updates self.next_sample if it is time to update.