change 'electrode_group' col of Units to be ElectrodeGroup type

[bendichter]

Motivation

fix #834

Now electrode groups can be added to the Units table

Checklist

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[codecov]

Codecov Report

Merging #820 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #820   +/-   ##
=======================================
  Coverage   73.72%   73.72%           
=======================================
  Files          60       60           
  Lines        7190     7190           
  Branches     1517     1517           
=======================================
  Hits         5301     5301           
  Misses       1459     1459           
  Partials      430      430

Impacted Files	Coverage Δ
src/pynwb/misc.py	86.27% <ø> (ø)	⬆️

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[bendichter]

@ajtritt @oruebel this is part of NeurodataWithoutBorders/nwb-schema#248, but it also is needed on a few other fronts. Was making the electrode_group of type "array" intentional? It seems like it should be ElectrodeGroup.