DESIGN

For now, everything is in 2D.

Two coordinate systems in use: body-fixed and world-fixed. Body-fixed
coordinates are given in units relative to some body's origin--the body's
orientation has no effect on coordinates in this system. World-fixed
coordinates are given in units relative to some notional world origin.

Angles are in radians, measured in a standard unit circle fashion: 0 radians is
due east, pi/2 due north, pi due west, 3*pi/2 due south.

Spacecraft have the following attributes:
- physical body, used for physics calculations
- graphical body, which may be the same as the physical body, used for
  graphical representation
- thrusters

Physical bodies have the following attributes:
- mass
- density map? (texture where transparent = 0 mass, darkest color = max density
  and light colors are somewhere in between. Maybe make it monochrome, so there
  are only 256 values to choose from?)
- center of mass, calculated from ship's density map, if any
- moment of inertia calculated around the center of mass
- orientation around the z-axis passing through the center of mass (scalar)
- linear velocity
- linear acceleration
- angular velocity
- angular acceleration
- shape used for collision detection
** For now, assume center of mass is static and is the same as the origin. Later
   we can tackle the situation where the COM might be dynamic.

Graphical bodies have the following attributes:
- origin
- shape
- color/texture?

Thrusters have the following attributes:
- point of application on body, in body-fixed coordinates
- force vector relative to un-oriented body
- key binding

Shapes have the following attributes:
- list of vertices relative to the body's origin (be it an arbitrary origin or its center of mass)

Types of dynamic influences:

Thruster
Tractor
Gravity
RubberBand