inconsistency between "Slot.fromDict" and "Slot.__setstate__"

there is a minor difference between the implementations of Slot.fromDict and Slot.__setstate__ which is usually irrelevant, but produces different configurations when combined with #12 (closed) and lhcb-core/LHCbNightlyConf!810 (merged).

When a data project appears twice in the lot configuration, Slot.fromDict(slot.toDict()) effectively hides the second copy, but pickle.loads(pickle.dumps(slot)) preserves both of them, with the net result that starting a buggy slot from the frontend resulted in a configuration that fails to checkout the data packages (because it tries to check them out twice).

With the fix to #12 (closed) and lhcb-core/LHCbNightlyConf!810 (merged) the discrepancy becomes irrelevant, but I think we should anyway harmonize the two implementations.