Allow exporting edf where a channel contains only constant values

[hofaflo]

Currently, exporting to edf fails if one channel contains only constant values and physical_range="auto", because edfio requires that (explicitly passed) physical minimum and maximum differ from each other (link). This requirement is suggested in Q8 in the EDF FAQ, because min == max leads to a division by zero when calculating the signal offset, which might cause an error when loading the file. This PR ensures that physical minimum and maximum are always different when using physical_range="auto". For physical_range="channelwise", edfio takes care of that , as mne passes no physical range to edfio.EdfSignal.

[cbrnr]

LGTM, thanks @hofaflo! Could you add a changelog entry please?

Just out of curiosity, what are the implications of having a pmax that is actually not the true maximum of the signal? I mean, the test shows that everything is correctly saved and loaded, but then pmax in the file is just wrong?

[hofaflo]

what are the implications of having a pmax that is actually not the true maximum of the signal?

None that I know of. The physical range (together with the digital range) is just used to determine signal gain and offset. There is no requirement that they actually occur within the signal. This is already the case for physical_range="auto", as the min/max per channel type is used. In this specific example, every physical 0 is stored as a digital -32767, and a physical 1 would be stored as a digital +32767, but there are none of these, so it doesn't really matter.