Source flags needs to handle if the planet doesn't cross the scan

[msilvafe]

The preprocess method SourceFlags calls coords.planets.compute_source_flags which currently fails if the planet is not present in the observation. We need to run this source flagging step on all CMB observations because we let the moon and other SSOs (Jupiter, Saturn, Mars, Venus, etc.) cross through our CMB field but we don't want them binned into the maps. However the planets are only present in a few observations. So we either need to call planet.coords.get_scan_q() in SourceFlags to check if the planet is in the observation first before calling compute_source_flags or add the logic into compute_source_flags after the call to get_scan_P.

[msilvafe]

@RemingtonGerras pointed out coords.planets.get_nearby_sources perhaps we can use that to do the logic check in SourceFlags