Fix neuroexplorer version 106

[h-mayorquin]

NeuroExplorer has written .nex version 106 since 2011, and the reader cannot open those files when a waveform variable has a prethreshold time. This PR enables that. Version 106 added the prethreshold time, the part of each waveform snippet recorded before the threshold crossing, stored as the float64 PrethresholdTimeInSeconds.

So now:

• Version 106 files with a non-zero prethreshold time open, where before they raised UnicodeDecodeError.
• wf_left_sweep carries the file's prethreshold time in samples (8 for 0.0002 s at 40 kHz) instead of a hardcoded 0.
• Files before version 106 still report a left sweep of 0, since those bytes are padding there.

For tests I added nonzero_prethreshold.nex, a version 106 file already on gin, cut from the vendor sample. Critically, the compliance tests were not running on any NeuroExplorer file because the attribute was misnamed: it was files_to_download, and I changed it to the correct entities_to_test.