leaves out of the avoidnodes table, there's really no reason to have them
there and having them causes too much spacing between trees. Reduced the
prevalence of sequoias.
Most of the added code written by RealBadAngel
Also adds a couple of additional biome controls to jungle trees and conifers.
Also adds a text file describing the biome settings in a human-readable manner.
At present, this doesn't actually add anything new, it just refactors the
code to allow for more trees, and adds a bunch of biome definitions, tree
models, textures, etc. but no code to use them, yet.
