prune_current_assignments() should also clear considered_destinations

[dgets]

If ShipHistory is to be fully integrated, it will be necessary to have prune_current_assignments() also go into glo.Variables.considered_destinations, and wipe out any destinations that the ship was previously considering. This would require either modification of considered_destinations from a list into a dict, in order to store things along with the ship.id, or else a considered destination field could be added to the ShipHistory class, enabling it to be tucked in along with everything else that belongs there (this is the way I'm going to try to handle it).