narrow the definition of a scalar dataset

[lawrence-mbf]

Fixes #430

How to test the behavior?

Export the following timeseries:

ts = types.core.TimeSeries( ...
            'data', 12.3, ...
            'description', 'XYZ', ...
            'data_unit', 'time', ...
            'timestamps', 12.3);

However you validate it, the space must be using a non-scalar space specification.

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

Merging #431 (d4a3bc7) into master (8fc2589) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   85.97%   86.01%   +0.03%     
==========================================
  Files         125      125              
  Lines        5070     5070              
==========================================
+ Hits         4359     4361       +2     
+ Misses        711      709       -2     

Impacted Files	Coverage Δ
+io/writeCompound.m	81.48% <ø> (ø)
+file/isShapeScalar.m	87.50% <100.00%> (ø)
+file/fillExport.m	98.47% <0.00%> (+1.52%)	⬆️

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