Preserve identity on Probe.copy()

[h-mayorquin]

Closes #421. Probe.copy() was dropping contact_ids, shank_ids, contact_sides, annotations, and contact_annotations; the only exclusion that was ever intentional is device_channel_indices (wiring, not identity). I fixed it by routing copy() through the existing to_dict / from_dict pair, which is already the canonical enumeration of a probe's serializable state via _dump_attr_names, and popping device_channel_indices from the intermediate dict. This gives one source of truth for what a probe "is" and means any attribute added to _dump_attr_names later will automatically round-trip through copy().