GDF reader bug about mne-python

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[cbrnr]

Note that I get the following warnings when loading the file:

RuntimeWarning: Channels contain different highpass filters. Highest filter setting will be stored.
RuntimeWarning: Channels contain different lowpass filters. Lowest filter setting will be stored.

[larsoner]

We could either restrict info["lowpass"] to be strictly less than zero or (my vote) improve the sufficient-lowpass check to only run if info["lowpass"] > 0

[cbrnr]

Whatever you end up deciding, I just confirmed with @schloegl that the file in question does indeed have all zeros for its filter settings, so we need to deal with such files gracefully.

[larsoner]

Yeah if we know this can happen with GDF files on some systems we could detect the behavior for at least that format

[cbrnr]

I think even with EDF it is not guaranteed that those header fields contain sane values. We could use fallback values of 0 Hz and fs/2 Hz in such cases, respectively.