function FrictionEquation(bodyA, bodyB, slipForce){
    Equation.call(this,bodyA, bodyB, -slipForce, slipForce);
    this.ri = new Vec3();
    this.rj = new Vec3();
    this.t = new Vec3(); // tangent
}
Constrains the slipping in a contact along a tangent
@class FrictionEquation
@constructor
@author schteppe
@param {Body} bodyA
@param {Body} bodyB
@param {Number} slipForce should be +-F_friction = +-mu * F_normal = +-mu * m * g
@extends Equation