ShanksElectrode coordinates should be optional

[pauladkisson]

Currently rel_x, rel_y, and rel_z are mandatory fields on ShanksElectrode. But sometimes labs don't have these coordinates, and that can cause problems downstream for Spyglass -- see LorenFrankLab/spyglass#1345.

To fix this issue, I propose that the coordinates in ShanksElectrode should be optional.