Fixed error in length calculation (had a hardcoded value that assumed 6 minutes)
Created SpeedVectorGraphic to allow reuse of the COG/speed vector in other graphic components (e.g. VesselOutlineGraphic, VesselTriangleGraphic). Added code that dynamically manages the length of the speed vector based on the map scale.
Small changes to accomodate a change in SpeedVectorGraphic (now using a VesselPositionData instead of a VesselTarget)