Schedule should have the option of referencing Harp Time

Currently, the schedule computes future task changes based on the local time. It would be valuable to configure the schedule to run off of Harp time instead of local time if we care more about periodic events happening on a synchronized global time reference rather than intervals. (i.e: absolute times more important than relative times). This feature would also guarantee that multiple cuTTLfish boards could trigger synchronously in a way where they never drift from each other.