SE3 prototype - hopefully according to current structure concept

NOT READY FOR MERGE
- not complete
- not correct
- just to have something to discuss / (dis)agree on

New here : SO3 and T3 are seen as speceial cases of SE3 (subgroups) and therefore children!

(runs fine with non optimized but crashes currently with optimized code - no time to debug, right now)
Should probably be `Vector6<Scalar>`