keep time with external guidance but match retrigger start point

[wraybowling]

Created from #5 This might be resolved with #9 or even with #11.

The idea was that the clock input might be coming from one module and that the user might want the behavior of the retrigger to be one of two modes

• time is always kept with the incoming clock
• time is kept with the retrigger

but if the incoming clock is not perfectly in sync, then the clock cannot reliably tell us when to trigger high. The retrigger offset would need to be added to that timestamp.

• research if anyone would even want this