Fix bug when exporting HDF5 datasets with unlimited dimension

[oruebel]

Motivation

Fix #154

When an HDF5 file contains datasets with unlimited dimensions, use of the hdmf.utils.get_data_shape method yields the wrong data shape because the functions looks at maxshape first. This leads to an error on export because Zarr cannot create a dataset with undefined dimensions.

How to test the behavior?

See #154

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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• 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-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (71bfa6d) 85.07% compared to head (21d5a6d) 85.16%.

Files	Patch %	Lines
src/hdmf_zarr/backend.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #155      +/-   ##
==========================================
+ Coverage   85.07%   85.16%   +0.08%     
==========================================
  Files           5        5              
  Lines        1146     1146              
  Branches      311      311              
==========================================
+ Hits          975      976       +1     
+ Misses        115      114       -1     
  Partials       56       56              

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