support negative times for CTF files

So I didn't know this but it seems like CTF has the ability to start its recordings with negative times by using a sync trigger and asking the scanner to include a baseline period.

Other packages Brainstorm (and presumably fieldtrip) respect this field and adjust their time arrays accordingly.

MNE reads in the required field
https://github.com/mne-tools/mne-python/blob/855edbf48835bcee80e773eb7047d0a4caa14a1c/mne/io/ctf/res4.py#L126

and adjusts annotations and bad_segments to account for it.
https://github.com/mne-tools/mne-python/blob/855edbf48835bcee80e773eb7047d0a4caa14a1c/mne/io/ctf/ctf.py#L147-L156

That MNE shifts other stuff and not the time array leads me to believe that MNE can't have negative time values in raw objects. Is that true?

This cropped up because I was doing some custom event marking for an experiment using mne and the resulting events were read into brainstorm and were all shifted by 1 second. In this particular case, I'm time shifting the events before I save them out, solving my immediate issue but it still seems a bit like a bug?

-Luke

Beginne mit der Behandlung des CTF-Feldes in mne/io/ctf/res4.py an der verlinkten Stelle und untersuche anschließend die Anpassungen an Annotationen und bad_segments in mne/io/ctf/ctf.py. Verfolge, wie sich der Basiszeitraum auf die Zeitwerte der Rohdaten und die Ereignisausrichtung auswirkt. Erledigt ist die Aufgabe, wenn CTF-Aufzeichnungen mit negativen Startzeiten konsistente Zeit-Arrays, Annotationen, bad_segments und Ereignisse beibehalten.