- in class CollisionTraversalNodeBase and derived classes.

  sqrt should be applied after method call.

  enable_distance_lower_bound.

  - add a Boolean value in CollisionRequest to trigger this option,
  - if selected, fcl::collision function will compute a lower bound
    on the distance between the two objects if they do not collide.

   This fixes test_fcl_simple.

- The convention of FCL is object1 --> object2
- mesh-mesh:   object1 --> object2 (correct)
- shape-shape: object2 --> object1 (fixed)
- shape-mesh:  object2 --> object1 (fixed)

  Matrix3fX::isIdentity,
  Vec3fX::norm, Vec3fX::squaredNorm,
  Vec3fX::setZero,
  operator * (double, Vec3fX).x