Add MoonTargetSeparationRange constraint

[ejeschke]

This is a constraint on the distance from target(s) to the moon. It would specify a minimum and maximum angular separation. Either minimum or maximum can be None, in which case there is no constraint in that direction. (Typically we might assume that only a minimum separation would be specified, but I know of engineering items that would require a maximum as well).

This is dependent on having a reliable way to get the moon position and calculate the angular separation.

[bmorris3]

MoonSeparationConstraint was added in #61.