add function to brute force test the distance function

[UnixJunkie]

To try finding a counter example to the triangular inequality when you have doubts about the distance function

[UnixJunkie]

val test_distance: int -> P.t array -> (P.t, P.t, P.t) option

[test_distance nb_tests points] return None if no violation was found, Some (x, y, z) else