Add support for accessing Biome information stored in an Anvil chunk

Added support for accessing and modifying biomes.  Changed IChunk to add support for the AnvilBiomeCollection class, which is just a wrapper around the ZXByteArray that stores biome info.  AlphaChunk.cs just stubs the getter for this to return NULL, since alpha biomes are mathmatically generated at run-time, and cannot be edited.