$LastChangedDate: 2010-04-14 13:55:46 -0400 (Wed, 14 Apr 2010) $

The representation of the simulated world is factored into:
- World class (top-level)
- Planet class (flat or spherical)
- Conversion functions (in world::conv namespace)

The World and Planet classes are integrated and together
define the simulated world but do not contain 3D objects (SceneGraph does).
Planet is a private implementation of the World class.

The interface of World was designed for either a flat or spherical world,
but this fact should be kept internal to the World class if possible.

The world::conv conversion functions convert between physical metrics,
spherical coordinates, and graphical coordinates.
