speed up electrodes import

[magland]

Description

• Fixes redundant time-consuming insertion of electrodes during import #1121: Reduces number of fetches
  • common_ephys.py: Introduce a region_ids_dict so we don't make redundant calls to the database

Checklist:

• This PR should be accompanied by a release: no
• N/A. If release, I have updated the CITATION.cff
• This PR makes edits to table definitions: no
• N/A. If table edits, I have included an alter snippet for release notes.
• N/A. If this PR makes changes to position, I ran the relevant tests locally.
• I have updated the CHANGELOG.md with PR number and description.
• N/A. I have added/edited docs/notebooks to reflect the changes

[magland]

I tested this and it seems to have resulted in the desired inserts into the Electrodes table, and the run time for that step was around 20 seconds compared to 14 minutes.

[CBroz1]

Thanks for this, @magland !!