Fix edges of 2d-histogram in UnitWaveformDensityMapWidget

[ecobost]

Currently, sw.plot_unit_waveforms_density_map(analyzer, unit_ids=[0], use_max_channel=True) produces this:

The density map has some spurious high values on the very first row (more visible near the peaks) and also on the bottom although less visible in this example.

The issue comes from the clipping in

spikeinterface/src/spikeinterface/widgets/unit_waveforms_density_map.py

Line 129 in 5a58beb

wf_bined = wf_bined.clip(0, bins.size - 1)

that counts any value with amplitude less than bin_min as belonging to the first bin and any value above bin_max as belonging to the last bin of the histogram, thus producing those lines on the plot.

This PR uses equal-sized bins from bin_min to bin_max and uses np.histogram to count occurrences. It produces:

without any artifacts. (Also tested it with multiple channels and multiple units)

Granted, it looks uglier but it's more correct. And it will look better for nicer data, this is a 2-minute recording just for testing.