Clean Up VesselID Prefix and URL/UUID Concatenation

PR !171 (merged) sets the simulator vesselID from the .env file, and PR !173 (merged) uses it for SignalK JSON according to the SignalK spec (self as key for vessel in vessels).

Further cleanup proposed:

• Correctly concatenate strings for self to that the uuid env parameter can be only the uuid itself. Also fixes vessel URLs/UUIDs accordingly.
• Audit spec for usage of the vessels. prefix in self parameter for the full sources and in context parameter for delta sources and change accordingly.