Immediate recalculation of Mass, Center and AABB on cluster change

solves #151 in its own way

* Mass, Center and AABB are adjusted on add/removeVoxel
* the AABB moves from CollisionDetector to VoxelCluster so that it is closer to the data it operates on. 
* The AABB-recalculation is solved using 3 binary search trees (std::set), sorted along each of the axes. (AutoTimer claims that's fast enough. <1us most of the time)
* There is no need of finishIntialization() anymore, everythng is valid, all the time.
* Voxel::mass renamed to Voxel::normalizedMass, as it is multiplied with a scale when used in Physics.

* delta_sec becomes deltaSec everywhere
* minor refactorings to comply to coding-standard