Add "bounds" to SpatialSeries fields

[rly]

Motivation

I noticed that pynwb tests were passing with NWB schema 2.7.0 even though "bounds" was not added to SpatialSeries until after NWB schema 2.7.0, so really the IO tests should fail.

Until then, on the dev branch, "bounds" in SpatialSeries.__init__ will not be written to disk because it is missing from __nwbfields__.

Using the convenience test classes can hide this error. I think we should be more explicit when writing roundtrip tests to ensure that all expected fields are read correctly from the written file.

This PR merges into a new nwb_schema_2.8.0 branch.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (dc98e84) to head (9d967e6).

Additional details and impacted files

@@                Coverage Diff                @@
##           nwb_schema_2.8.0    #1907   +/-   ##
=================================================
  Coverage             91.85%   91.85%           
=================================================
  Files                    27       27           
  Lines                  2689     2689           
  Branches                701      701           
=================================================
  Hits                   2470     2470           
  Misses                  145      145           
  Partials                 74       74           

Flag	Coverage Δ
integration	72.62% <100.00%> (ø)
unit	83.30% <100.00%> (ø)

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