Add depth information to Neuropixels electrodes

[bparks13]

With v1.0.1 of the GUI, the LFP Viewer has the ability to color channels By Shank, but this will require us to add some information to the continuous channels. Specifically, we can add the X/Y position of the electrodes as they are represented in the ProbeInterface configuration, as well as the shank name as a Group in the channel.

NOTE: This will not work as-is, because currently the LFP Viewer cannot resolve two channels that have the exact same Y position, which some Neuropixels electrodes do. The PXI plugin gets around this by not reporting the true X/Y value of the channel, and always incrementing the electrodes so there is at least 1 micron difference between consecutive electrodes, but the next version of the GUI should fix this issue. Until that version is released, this will need to remain as an issue or a draft PR.

[bparks13]

note that Josh recently updated how this information is added to channel objects in the PXI plugin (open-ephys-plugins/neuropixels-pxi@107c5a7):

Shank
group.name = "Shank " + String(shankIndex + 1)
group.number = shankIndex

Electrode Position (X, Y)
position.x = True electrode X position
position.y = Must be a unique value for compatibility with the legacy LFP Viewer channel-sorting algorithm (see the linked commit for calculation details in the PXI plugin)
Add the true electrode Y position as Metadata with name ypos to the channel object (refer to the linked commit for more details)