WIP tris selection

cc @Eric89GXL @dgwakeman @agramfort 

will share a validation example soon.
@Eric89GXL this is the relevant line. The intended logic can be expressed as folows. Select the rows in input tris for which all three values are present in vertics. The result will give you something that matches the test that I implemented but not the correct one that you proposed. In other words, there are vertnos in a label that don't satisfy these criterion... thoughts?