[WIP] Support NWB Schema 2.9.0

[rly]

Add support for upcoming NWB schema 2.9.0. This is a staging branch and will be merged into dev when all the 2.9.0 changes have been merged into here.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 97.65625% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 94.95%. Comparing base (cba0ae8) to head (7d7667a). :warning: Report is 30 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/pynwb/ecephys.py	91.30%	0 Missing and 2 partials :warning:
src/pynwb/file.py	91.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2092      +/-   ##
==========================================
+ Coverage   94.83%   94.95%   +0.11%     
==========================================
  Files          28       28              
  Lines        2751     2832      +81     
  Branches      708      725      +17     
==========================================
+ Hits         2609     2689      +80     
- Misses         82       87       +5     
+ Partials       60       56       -4     

Flag	Coverage Δ
integration	74.54% <81.25%> (+0.38%)	:arrow_up:
unit	84.78% <86.71%> (-0.07%)	:arrow_down:

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