& gen algorithm, improved Pool documentation, migration from old .maps to .rlems

Added a new map format: .rlem, Run Length Encoded Map, which is inspired by Minecraft's map format. RLEMs allow large amounts of terrain to be stored in very little space. I had to reduce the number of tiles available to the world gen algorithm to support RLE (doesn't work well for continous values). The more tiles the world gen alg uses, the less RLE helps. The advantage of RLE is I can now store changes to Cell tiles without trying to come up with some fancy diff-based system to save space.