Enh/epochs dynamic table

[ajtritt]

Motivation

Converting Epochs to a DynamicTable will make it easier for users to add metadata about epochs. More specifically, users will no longer need to make an extension to add epoch metadata.

How to test the behavior?

nwbfile = NWBFile(...)
ts1 = TimeSeries(...)
ts2 = TimeSeries(...)
nwbfile.create_epoch(2.0, 4.0, ['first', 'example'], [ts1, ts2])
nwbfile.create_epoch(start_time=6.0, end_time=8.0, tags=['second', 'example'], timeseries=[ts1, ts2])
nwbfile.create_epoch(start_time=10.0, end_time=12.0, tags=['third', 'example'])

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 #682 into dev will decrease coverage by 0.55%.
The diff coverage is 73.25%.

@@            Coverage Diff             @@
##              dev     #682      +/-   ##
==========================================
- Coverage   75.27%   74.71%   -0.56%     
==========================================
  Files          58       58              
  Lines        6738     6775      +37     
  Branches     1378     1392      +14     
==========================================
- Hits         5072     5062      -10     
- Misses       1273     1320      +47     
  Partials      393      393

Impacted Files	Coverage Δ
src/pynwb/form/data_utils.py	77.77% <100%> (-7.57%)	⬇️
src/pynwb/io/epoch.py	100% <100%> (ø)	⬆️
src/pynwb/ecephys.py	97.27% <100%> (ø)	⬆️
src/pynwb/form/build/builders.py	87.5% <25%> (-1%)	⬇️
src/pynwb/form/spec/spec.py	79.42% <40%> (-1.05%)	⬇️
src/pynwb/form/build/map.py	75.19% <43.47%> (-0.9%)	⬇️
src/pynwb/io/core.py	73.8% <61.53%> (-6.2%)	⬇️
src/pynwb/epoch.py	70.23% <70.9%> (-4.57%)	⬇️
src/pynwb/file.py	82.53% <75%> (ø)	⬆️
src/pynwb/core.py	70.16% <96.49%> (+1.13%)	⬆️
... and 4 more

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