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, this is exactly the kind of construction side. 

* This version is the concept where an SE3 object always keeps its interface (Derived), SE3<Derived> would mean loosing the interface (special functions of Derived will not be exposed anymore). 

* I proposed there a different fashion, where it would return just some SE3<Policy> but here Policy is not even the interface anymore. The interface would then be there only as provided by the user - he would choose the interface, just as he needs it, via .cast or cast cast construction (s. https://github.com/ethz-asl/kindr/issues/50#issuecomment-64553651). I'm almost done with that change and will commit the essential diff to this version soon.