Issue 584: Add warning on export if some attributes are not written to file

[ehennestad]

Fix #584

Motivation

A subset of attributes will get dropped on nwbExport. These are attributes that are required, but depends on a dataset. When the dataset is empty, these values are not exported, but that happens silently. This fix adds a warning message on export if such cases are present.

How to test the behavior?

tests.unit.nwbExportTest %(testExportDependentAttributeWithMissingParentA)

Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.63%. Comparing base (c51099f) to head (dd2215a). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
+ Coverage   90.60%   90.63%   +0.03%     
==========================================
  Files         106      106              
  Lines        4671     4687      +16     
==========================================
+ Hits         4232     4248      +16     
  Misses        439      439              

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