Support negative channel numbers

[gyger]

Swabian Instruments has introduced negative channel numbers in there raw file format for falling polarities, because one can record both signal directions at the same time now. Seems to not work in ETW currently.

[linzuzeng]

I think you can set, for example clip1.CHANNEL_OFFSET=11 to a Clip, if -10 is the smallest channel number on the negative side.

Do you think we should also support negative chn number?

BTW, Sorry for the late reply. My signal bridge is down LOL.