Bottom up octree build - write index

* Write octree index at end of hcloud file.
* New hcloud header format: write offsets rather than sizes for index
  and data segments.
* New index format: use 32 bit ints for counting voxels and points in
  nodes.
* Update viewer object to support new format
Point picking for HCloudView

This required an update to the geometry API to pass the current camera
position.