Finer customisation of Dawn/Dusk times
Northern users requested dawn and dusk to be configurable.
Preliminary use case would be to provide an offset to astronomical dawn and dusk values for the running day, so that narrowband imagers may start sooner and finish later.
The hour offset value is already implemented in Scheduler, but the angle at which we consider the astronomical event should be customisable. In northern locations, the Sun may never go lower than the astronomical dawn/dusk limit, or may take a long time to attain the limit.