Add lower bound on distance in collision detection (preliminary)

  - 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.
Adding parameter to track distance lower bound between BVH (preliminary).
Add documentation.
Add squared distance lower bound as a reference parameter.
Add a test on distance lower bound.
Add Boolean parameter in MeshCollisionTraversalNode derived constructors.

  enable_distance_lower_bound.
Replace TriangleDistance::triDistance by sqrTriDistance.

  sqrt should be applied after method call.
Add return value sqrDistLowerBound to leafTesting method

  - in class CollisionTraversalNodeBase and derived classes.
Unimplement specific collisionRecurse for MeshCollisionTraversalNodeOBB*

  - throw exception if called.
Remove abusive abort.