Add support for NWB schema 2.7.0

[rly]

Squash and merge NWB schema 2.7.0-related PRs to this branch.

When ready to merge:

• [x] Update the nwb-schema submodule to the 2.7.0 tag
• [x] Update changelog
• [ ] Do not merge using Squash and Merge so that individual PRs are kept as separate commits.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.81%. Comparing base (270291a) to head (02702a7).

Files	Patch %	Lines
src/pynwb/file.py	90.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1820      +/-   ##
==========================================
- Coverage   91.96%   91.81%   -0.15%     
==========================================
  Files          27       27              
  Lines        2663     2627      -36     
  Branches      686      688       +2     
==========================================
- Hits         2449     2412      -37     
  Misses        142      142              
- Partials       72       73       +1     

Flag	Coverage Δ
integration	72.47% <41.66%> (+0.11%)	:arrow_up:
unit	83.82% <95.83%> (-0.26%)	:arrow_down:

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[rly]

The NWB Tutorial at Cosyne is this Thursday. In case adding support for NWB schema 2.7.0 causes any last minute issues in PyNWB, MatNWB, or other tools, like NWB GUIDE, I would like to hold off of merging this and making new releases until after March 4.