Default World Generator - Continental Shelf Threshold Default

Changed the continental_shelf_threshold default value, as beaches were simply too large.
Default World Generator - Remove Debug Code

Removed a debugging shim which was disabling domain perturbation.
Default World Generator - Offset Y

Added a random Y offset (to go with the similar X and Z offsets) so that the origin is unlikely to be plugged into any 3D simplex noise generation.

This was creating problems because of the way all values converge at the origin.
Default World Generator - Caves

Added cave generation to the default world generator.