add level of detail mode to octree

[agurvich]

an extension to #116, what if instead of having a single .ffly file associated with each node there were multiple levels of detail .ffly files with smaller and smaller decimations. rather than opening/closing nodes we would move them up/down in LoD. would need to keep track of each node's LoD (kind of easy) and then determine when to load additional files for each node (very hard).

would need:

• associating node particles with > 1 .ffly file (currently assumes all node buffer particles live in a single .ffly file
• associating multiple particle meshes with a single node
• new "opening/closing" criteria
• rewrite octree walk to walk until threshold LoD rather than until it hits a closed node

so in short, a lot of work. could be a good ~week or two project but should not hold up submission.