Support NWB Schema 2.7.0

[lawrence-mbf]

Fixes #569

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.65%. Comparing base (5396243) to head (c48b5c1).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
+ Coverage   88.52%   88.65%   +0.13%     
==========================================
  Files         104      104              
  Lines        4636     4637       +1     
==========================================
+ Hits         4104     4111       +7     
+ Misses        532      526       -6     

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[bendichter]

This looks right. I think all we really need to do here is run some tests to make sure read/write works between PyNWB and MatNWB