Add ported driftscan mode

Thanks to @tammojan for the work on Dwingeloo driftscan mode of operation, ported to newer style of auto-scheduler.
New argument --pointing, constrain passes to antenna beam radius (half beamwidth) and pointing direction. Examples: antenna pointing straight up, 30 degree radius: '-i 30', pointing south at horizon: '-i 30 180', pointing west and 45 degrees up: '-i 30 270 45'.
Added pass reduction to fit the min separation, added minimum pass duration to extend pass beyond angular_separation to keep the pass at least min_pass_durtaion long.
Results prints ArZ El AzS.

Signed-off-by: Daniel Ekman knegge@gmail.com