add moon constraint to targets in fields.yaml

[fergusL]

At the moment we specify a one size fits all moon separation constraint that get used by the scheduler to veto any targets that are too close to the moon. We also have a problem where we want to observe some targets only when the moon is not up and others only when the moon is up.

If we add a minimum moon separation parameter to the observation config in the fields.yaml file we can specify moon separation>180?degrees and then pocs can prioritise dark time targets before the moon rises. Then when the moon has risen, those targets will be vetoed and pocs will start scheduling bright time targets. This mean we won't have to interrupt pocs in the middle of the night when the moon rises to switch target files.