1. Added a mid-western desert.
2. Added advanced options for World Age and Rainfall.

Tweaked terrain & feature gen for a less hilly/flatter (though still densely wooded) N. America.

Consolidation of terrain gen. for more jungle in C. America and a touch more marsh. Trivial/no impact.

Code tidy up allowing more natural wonders.

1. Lakes can be river sources
2. Terrain adjustments: more marsh; Central American rainforest generation

Tidy up terrain generation logic.

Added marsh. Turned off new Summer '17 patch default that adds large lakes. Already got The Great Lakes.

Minor adjustment to map generation to make boundaries between ice/snow/tundra/plains+grass to be more natural

Fixed bug where southern ocean tiles were impassable (like ice).

Minor reduction in forest cover for balance.