sEEG channel type

Following #1070 I would like to implement a channel type for stereotactic EEG. So far I've 

- added a fiff constant (which will pose a problem for other MNE software not recognizing it?)
- updated picking routines & viz defaults

The viz defaults are guesses which I will refine with some testing.

Are there other things that need to be done for a channel type?
honestly I fear we would overfit the usecase of marseille if we add these functions. I would rather try as much as possible to update the API of existing functions to facilitate your usecase. we can discuss it if you want.