Fix default ObjectMapping bug for DynamicTableRegion columns

[oruebel]

Motivation

The default mapping for DynamicTableRegion columns in the DynamicTable has a bug where we try to read the .table attribute from the VectorIndex instead of the VectorData dataset.

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style ? This can be checked running flake8 from the source directory.
• 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 #XXX notation where XXX is the issue number ? By including "Fix #XXX" you allow GitHub to close the corresponding issue.

[codecov]

Codecov Report

Merging #171 into dev will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##              dev     #171      +/-   ##
==========================================
- Coverage   70.03%   70.01%   -0.03%     
==========================================
  Files          30       30              
  Lines        5811     5813       +2     
  Branches     1368     1369       +1     
==========================================
  Hits         4070     4070              
- Misses       1305     1307       +2     
  Partials      436      436

Impacted Files	Coverage Δ
src/hdmf/common/io/table.py	55.55% <0%> (-3.27%)	⬇️

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