proposal: store 3D positions of probes e.g. electrodes

Currently, in the dandi-atlas, we need to process each asset and extract 3D electrode positions in a common reference image coordinates system. We store this in JSON files in the repo, which is working for now, but it means this data is not standardized or centralized into asset metadata for use across all DANDI applications. How would we feel about adding a field to incorporate this data? This would essentially need to mimic our [ndx-anatomical-localization](https://github.com/catalystneuro/ndx-anatomical-localization) extension, including information about the reference space and coordinate system, and a reference to what type of device is being localized.