Create new ElectrodesTable neurodata type

[rly]

Summary of changes

• Fix Electrodes table and frequency bands table should be their own types #336, will replace Define ElectrodesTable and FrequencyBandsTable types #337

Need to get PyNWB and MatNWB to work with this first.

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.

If this is the first schema change after a schema release (i.e., the version string in core/nwb.namespace.yaml does not
end in "-alpha"), then:

• Update the version string in core/nwb.namespace.yaml and core/nwb.file.yaml to the next major/minor/patch
  version with the suffix "-alpha". For example, if the current version is 2.4.0 and this is a minor change, then the
  new version string should be "2.5.0-alpha".
• Update the value of the version variable in docs/format/source/conf.py to the next version without the
  suffix "-alpha", e.g., "2.5.0".
• Update the value of the release variable in docs/format/source/conf.py to the next version with the suffix
  "-alpha", e.g., "2.5.0-alpha".
• Add a new section in the release notes docs/format/source/format_release_notes.rst for the new version
  with the date "Upcoming" in parentheses.

[mavaylon1]

@rly I can push this across the finish line because I was going to work on literally this due to some applications in TermSetconfig.

The rest of the work would be working this on the pynwb side, is there a PR for it?

[rly]

Bumping to 2.9.0 release.
TODO:

• Update version strings
• Test with MatNWB
• Test with PyNWB