Correctly apply camelCaps

There's still a lot of old code that doesn't follow this convention, but
lets make sure new code does!
Small improvements for BiomeGroup class

Improved the documentation, renamed a few methods, removed unused
getBiomes() method, implemented isAnalogousTo and made setGroupId
package-private, with better error checking.
Add proper default setting for biome groups

The rarities of the ice biomes still need to be tuned. Isles and borders
also need to be added.

Also, where is the Mesa biome? Probably special-cased by Mojang...