More work on the soil map loader. Removed a bunch of classes that were
part of an old idea I had about a month ago about the inner workings of
soil types. One class per tile type is not very effective, since it is
much better to attach desired properties to the tile. For example; it is
silly to rewrite the same code that slows a unit that walks over rubble
for every level of rubble clearance. It's much better to attach it as a
single property to every tile.