WIP tris selection

cc @Eric89GXL @dgwakeman @agramfort 

will share a validation example soon.
in a label.

```Python
ipdb> u
> /Users/dengemann/github/mne-python/mne/tests/test_label.py(440)test_stc_to_label()
    439     connectivity = spatial_tris_connectivity(tris)
--> 440     assert_true(connectivity.shape[0] == len(stc.vertno[0]))
    441

ipdb> len(stc.vertno[0])
3732
ipdb> connectivity.shape[0]
3729
ipdb>
```