Fix min/max chan amps for all-positive(/neg) data

[TomBugnon]

Hi Etienne and Raphael,

Currently, Sleep doesn't handle well channels with all-positive or all-negative values, due to the way the maximum and minimum values for display are derived from the channels minimum and maximum values.

I stumbled on this limitation as we load some non-EEG data into Sleep with all-positive values to assist with scoring: here I can't set the minimum value for the derivedEMG channel where I want

Here's a little fix that I think does the trick to manage channels with all-positive or all-negative values. There would be other acceptable ways of setting the bounds (eg self._yminSpin[i].setMinimum(-10. * max(abs(self['min'][i]), abs(self['max'][i]))) ), feel free to make changes if you see fit! I don't think this interacts in any way with other functionalities but I might be missing something.

Best, Tom

[TomBugnon]

up!

[raphaelvallat]

Looks great to me 👍