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?
I wrote 305, 59, 0 explicitly but if the testing file changes those numbers, the test breaks. If instead I the actual number of channels of each type on `fwd`, the test doesn't break... I'll change this to use `mne.io.pick.channel_indices_by_type`.