Math: test that trigonometric functions work with Unit base class.

The test fails.
Math: make trignometric functions working with Unit base class.

The test now passes again.
Primitives: added 2D circle primitive.
DebugTools: render boxes with size as half extent, not full.

Also clarified the documentation in Physics::Box and
Physics::AxisAlignedBox classes.
DebugTools: take also object rotation into account in ForceRenderer.

Currently the force was only translated, completely ignoring object
translation.
DebugTools: added support for 2D spheres into ShapeRenderer.