Considerations for new `observation-activity` state: `scan`

[danielczech]

For now I am treating the scan state in the same way as the track state: we wish to record data anytime the array is in either of these states. The scan state appears to refer to observations in which the array scans smoothly through changing RA and Dec values, as opposed to stop-and-stare observations at fixed RA and Dec (in the case of the track state).

As it stands, RA and DEC will update smoothly in the headers in the scan state. Are there any other considerations? Do we want to communicate to the processing nodes that the array is in the scan state or is the above sufficient?