Release NWB Schema 2.9.0

[rly]

Order of operations:

• Merge Re-add new ElectrodesTable neurodata type #624
• Merge FrequencyBandsTable #610
• Merge Add DeviceModel neurodata type #608
• Merge Allow EventDetection` to store channel index #620
• Merge improve doc of IndexSeries #614
• Implement and merge Implement NWBEP001 #625
• Review remaining issues but probably punt on addressing them to the next release since this one is delayed.

These PRs are chained onto each other so that we can test their functionality incrementally in PyNWB.

[rly]

Due to bandwidth limitations, we will push #625 from NWB Schema 2.9.0 to 2.10.0 which we will target for release at the end of June.