Make electrode table a DynamicTable

[ajtritt]

Motivation

The ElectrodeTable required users to make an extension to add additional metadata about electrodes. Making the electrode table a DynamicTable will avoid the inevitable plethora of extensions for adding new electrode metadata.

Additionally, the compound dtype of the ElectrodeTable would require unpacking every row of the table to query on a single column, resulting in poor performance for querying along columns.

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 ?

[codecov]

Codecov Report

Merging #634 into dev will decrease coverage by 1.49%.
The diff coverage is 73.86%.

@@            Coverage Diff            @@
##              dev     #634     +/-   ##
=========================================
- Coverage   76.84%   75.35%   -1.5%     
=========================================
  Files          59       58      -1     
  Lines        6392     6479     +87     
  Branches     1270     1299     +29     
=========================================
- Hits         4912     4882     -30     
- Misses       1143     1242     +99     
- Partials      337      355     +18

Impacted Files	Coverage Δ
src/pynwb/form/__init__.py	66.66% <100%> (-15.16%)	⬇️
src/pynwb/io/file.py	96.66% <100%> (ø)	⬆️
src/pynwb/ecephys.py	93.63% <100%> (-0.86%)	⬇️
src/pynwb/form/query.py	55.76% <100%> (+0.96%)	⬆️
src/pynwb/form/spec/spec.py	79.85% <27.27%> (-0.42%)	⬇️
src/pynwb/form/build/builders.py	88.4% <50%> (-2%)	⬇️
src/pynwb/form/backends/hdf5/h5tools.py	69.1% <71.42%> (-5.82%)	⬇️
src/pynwb/form/utils.py	88.33% <75%> (+0.09%)	⬆️
src/pynwb/core.py	71.29% <75%> (-1.19%)	⬇️
src/pynwb/form/build/map.py	76.96% <77.41%> (-2.12%)	⬇️
... and 12 more

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