- moved SDK init stuff to own internal static class

- added mutex for SDK
- made container fields of Instance readonly
- added synatic sugar for making native function calls with reasoncode handling
- added mutexes for instance locking
- added AvatarPosition creator for self coordinates
- changed from Color to Nexus ColorRGB
- made more Instance core methods chainable
- more documentation and clarification on thread-safe methods
- minor code cleanups