Resolves #54 - Restructure of occupancy bitmaps

[davids91]

It is better to have one occupancy bitmap cover multiple levels of the octree hierarchy so it is more effective to query for it when iterating the tree.

Brick occupancy bitmaps are removed, while node occupancy bitmaps are extended to u64. This in theory should make bitmap queries more effective when iterating the tree because:

1. There are way fewer nodes, then bricks since Implement Homogenous Nodes #29
2. One node covers a bigger area, than a brick, so major misses can be detected earlier
3. mapping an index and querying the bitmap in every step during brick traversal requires much more computation, greatly diminishing the performance gain of occupancy bitmaps