RoughRavines: Initial generator implementation.

This provides the basic shape of the ravines, with the basic settings based on GridStructGen, and good default values.
RoughRavines: More settings - size, width, roughness
RoughRavines: Added per-height radius modifier. Ledges!
RoughRavines: Made floor and ceiling settings-adjustable.

The world.ini has settings for the minimum and maximum height for each at the ravines' center and edges.
Added a missing include for abs(float).
Merge pull request #1252 from mc-server/RoughRavinesGen

Rough ravines gen