[ENH] tris selection

cc @Eric89GXL @dgwakeman @agramfort 

will share a validation example soon.
> if np.max(tris) != len(np.unique(tris)):

that's not the same. it could be true in many other conditions that are not relevant to the problem. I would stick with the exact test used in coo.py.

> tris_ = np.searchsorted(tris, np.unique(tris))

probably. we need a reasignment to the indices that keeps their order but has a value range between 0 and length