Delete BaseHeight property from random map JSON files and rmgen library, refs #6, #4973, r8494, r9096.
The height is only a concern to the random map script, not the rmgen library, nor the gamesetup, nor the simulation.
Reveals the actual initial elevation to the random map script author without looking up the JSON file.
Group and rename all height constants of random maps below the template names.
Rename waterHeight to heightSeaGround to prevent confusion with the actual waterlevel.
Remove useless paintTerrainBasedOnHeight calls on African Plains and Polar Sea.