Added a unit test for the Map.Assign method
Made the MapVM.Map property read-only and added a constructor argument for setting it initially
Implemented System.Collections.IEnumerable.GetEnumerator in ObservableDictionary
Removed the setter from the Map.BlockedCells property.  The collection is modifiable, but it will always be the same collection instance for the lifetime of the map.
Added a test to verify that the pathing commands' states are updated when path-finding completes; Ignored for now because it won't pass