Add DBS (deep brain stimulation) as new channel type

[richardkoehler]

Reference issue
Related to #8681.

What does this implement/fix?

Add DBS as new channel type for recordings obtained from deep brain stimulation electrodes.
The channel type DBS is referenced to in iEEG-BIDS (https://www.nature.com/articles/s41597-019-0105-7

[larsoner]

1. change test_constants.py hash to the one in latest fiff-constants master (you will have test failures because of this currently)
2. git grep -i "seeg" and go through the results to see what things you're likely going to have to change to add a new EEG-like channel type. Not all of them will need to be replicated but many will.
3. Bonus: If you have some DBS data you could anonymize and share we could make an example out of it, which would be cool! But even without this, you could just modify the existing sEEG to mention that DBS electrodes can be processed similarly.

(2) will be a long list but hopefully won't require too much time to actually get through!

[richardkoehler]

I had to create a new pull request (as I switched branches): #8739