Adjusted river generation defaults to simulate complex Amazonian river system. (i.e more rivers)

Code tidyup. Trivial/no impact.

Reduced the number of large lakes added by default in Summer '17 patch.

Adding marshes. Also minor improvement to FeatureGeneration logic for balance.

Added Patagonia.

Very minor. Moved Amazon (center) a little higher. V. slightly nerfed rainforest coverage.

Rainforest generation more algorithmic and random for balance. Less OP.

Added desert to terrain mix for balance and realism. (E.g: Patagonia)