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?
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (dc98e84) to head (9d967e6).
Report is 1 commits behind head on nwb_schema_2.8.0.

Additional details and impacted files

@@                Coverage Diff                @@
##           nwb_schema_2.8.0    #1907   +/-   ##
=================================================
  Coverage             91.85%   91.85%           
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  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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[stephprince]

looks good to me!