Add a smart ending to schedules

[kmharrington]

Right now we give it a time and it ends by that time, but that's not ideal when a CMB scan could span that end and we don't feel all that strongly about the time +/-30 min.

We need an option to close out a scan or field if it's within some buffer time for an end time. This way we can optimize where the edges of schedules can be are improve our observation efficiency.

I think this should be implemented during or after #78