WIP tris selection

cc @Eric89GXL @dgwakeman @agramfort 

will share a validation example soon.
Sorry, was 2 lines too early. Deleted + reposted.

for some reason this one fails if I test against stc.vertno[0] (3 more points)
I'm not sure whether this is expected or not.

thoughts @agramfort @Eric89GXL @mluessi ??