Rename electrode coordinate columns from x/y/z to ap/dv/ml

[h-mayorquin]

The ElectrodesTable in the NWB schema defines brain coordinates using generic x, y, and z column names with a PIR (Posterior, Inferior, Right) sign convention. However, the mapping between x -> ap, y -> dv and z -> ml is not obvious from the name of the columns. I have been working with anatomical localization and I find myself coming back to the schema in order to get this right. I don't think that there is an upside of having abstract coordinates x, y and z unlike ndx-anatomical-coordinates where the axes are arbitrary.

I propose renaming them to ap (anterior-posterior, + posterior), dv (dorsal-ventral, + ventral), and ml (medial-lateral, + right/lateral), preserving the existing sign convention with no data transformation required. Backward compatibility can be handled at the API level in PyNWB and MatNWB: when a user accesses ap on an old file containing x, the API returns x transparently, and vice versa. The schema would deprecate x, y, z in favor of ap, dv, ml.