[Backport #5841] EDF stim_channel

[massich]

This PR backports #5841.

It mainly adds the same test to show its behaviour, and changes the warning doc.

[massich]

I'm not quite sure what to do here.

If we call raw = read_raw_edf('test_stim_channel.edf', stim_channel=None)
where 'test_stim_channel.edf' has only 3 channels named: 'status', 'trigger', and 'sine 1 Hz' it tries to something with channel name called 'EDF Annotations' whatever... which seems really wrong.

E           RuntimeWarning: 1 channel names are too long, have been truncated to 15 characters:
E           ['EDF Annotations-1']

This is not the case in master (obviously 'cos the code is completely different).
So I guess the question is: Should we solve all these bugs? or tell people to use 0.18? Or should we just copy paste whatever is in edf.py for version 0.17.1 ?

[agramfort]

LGTM

thx @massich

[agramfort]

@larsoner please merge if happy

[larsoner]

Thanks @massich