Force/constraint interface.
author  jeffrey traer bernstein (original source code)
author  Johann Sorel (adapted to unlicense project)

The difference between forces (spring,...) and constraints (distance,rotation,...)
is order. Constraint forces are applied after normal forces.

• forces could be applies in a random order, they are more or less independant
• Constraints are absolute, they are used to compensate the forces applied.

Is this force active.
return  true if active, false otherwise

Set active state.
param  enable

Apply force.