Support adding string type as value to neurodata object properties

[ehennestad]

Motivation

Adding string types to properties causes error on nwbExport. As the string type was introduced in R2016b, it should be supported by MatNWB.

How to test the behavior?

nwb = tests.factory.NWBFile();

generalExperimenter = ["John Doe", "Jane Doe"];
generalExperimentDescription = "Test with string data types";
nwb.general_experimenter = generalExperimenter;
nwb.general_experiment_description = generalExperimentDescription;

nwbExport(nwb, "test.nwb");

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?

[ehennestad]

NB:
2 tests are failing because the Clustering neurodata type was deprecated in pynwb, but is still used in the matnwb test suite

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.33%. Comparing base (86c3a9e) to head (be6b258).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #675   +/-   ##
=======================================
  Coverage   95.33%   95.33%           
=======================================
  Files         139      139           
  Lines        5311     5315    +4     
=======================================
+ Hits         5063     5067    +4     
  Misses        248      248           

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