Slight refactor to Vector Index get

[mavaylon1]

Motivation

hdmf-dev/hdmf-zarr#237
Using to_dataframe in Zarr is slower than HDF5, most likely due to how we handle references. The change is gathering all the data at once to resolve, rather than multiple I/O reads iteratively.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.56%. Comparing base (bcde246) to head (f68969c).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1248      +/-   ##
==========================================
+ Coverage   91.55%   91.56%   +0.01%     
==========================================
  Files          42       42              
  Lines        9539     9552      +13     
  Branches     1926     1928       +2     
==========================================
+ Hits         8733     8746      +13     
  Misses        523      523              
  Partials      283      283              

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[alejoe91]

@mavaylon1 tested again on my end an this dramatically speeds things up :)

[rly]

This looks good to me, but see comment above