Ditched Chunk Spanning Algo

The chunk spanning gen code turned out to be unnecessary. The real
problem was that the Lake gen code was poorly balanced. That's fixed
now.

I've also added some code to prevent Wells and Lake from spawning on
overly rough terrain. No more wells on the edges of cliff faces for
example. I had to rebalance the spawn chances though. Of note is the
fact that I increased the chance multiplier for surface deposit biomes
from 1.5 to 4.  Distribution balance may need some more work.

Additionally, I've started work on an Oil Field biome, a rare biome that
spawns massive amounts of Oil. Its currently disabled, but the ground
work is there. Next step is creating a suitably sparse Simplex noise
field for the biome replacer.