change dims for Units waveform

[bendichter]

add an optional 3D shape for waveform_mean and waveform_sd including a third dimension for electrodes

Motivation

You can easily put multi-electrode waveforms in the waveform_mean and waveform_sd columns, but it previously was not clear what the order of those dimensions was. This adds an optional 3D shape to waveform_mean and waveform_sd that puts electrodes in the last dimension

How to test the behavior?

Show here how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style ? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number ?

[rly]

Could you give an example use case for the new electrodes dimension?

[bendichter]

It has come up several times that neurons are recorded from more than one electrode. Most recently with neuropixel data

[rly]

Ah, thanks. Right. A unit recorded on multiple electrodes has a mean and sd waveform on each electrode, and it makes sense to store those.

[bendichter]

@kristinquick this has been merged into dev