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)
Yes, but this (above) is kind of your idea of an modeling oriented interface: the factories are just there to achieve the intended construction based on model ideas, without too much risk of doing it wrong. So maybe you give it a minute, if this is what you had in mind (with this comment https://github.com/ethz-asl/kindr/issues/52#issuecomment-63226459).