Adding stability to v-structure and orientation rules - constraint based discovery

[adam2392]

I was running some exps and noticed that we do not have implemented order stability at the level of orienting edges. The constraint based skeleton discovery is order-independent by default, but we should also impose order-stability for colliders and orientation rules.

This is described in https://jmlr.org/papers/volume15/colombo14a/colombo14a.pdf