Cluster cache

This PR is against physics_test and introduces
* ClusterCache, which caches lists of voxels which can be added to any VoxelCluster via fillCluster()
* ClusterLoader now fills voxel-lists instead of clusters (use ClusterCache)
* VoxelCluster::addVoxel() now expects a Pointer because VoxelCluster now holds Pointers

this can be merged at any point by @xchrdw and is intended (as test of github xD) plattform for discussion and to keep an overview