[VECGEOM-431] Fix to box::DistToOut() for points on surface and exiting the box

Fix to a bug pointed out by Evgueni:

Box(10,10,10), DistantanceToOut(p,v):

Point(10 + 1e-12, 0, 0) Vector( 1, 0, 0) 'leaving shape' reply is -1, should be 0