This PR introduces the start_position argument to seq.scan(). This optional argument, if passed, will move the telescope to the start position before beginning the scan.

One thing to consider here is the time checking. In each scan we are checking whether the stop_time has already passed and whether there is enough time between "now" and stop_time for the scan to be at least min_duration. In either case we just skip the scan if there isn't enough time.

Now the stop_time passed check happens first, then motion to start_position, and then we check if there's enough time left. I don't know how tight the scans could possibly be scheduled, but since we previously would already be in the correct position at the start of the scan I could potentially see an impact on whether we pass the min_duration check given we'll be taking up some time to move that was maybe previously accounted for in the scheduler. Someone more familiar with the scheduler should comment.

Resolves #190.