add mock icephys and tests

[bendichter]

How to test the behavior?

from pynwb.testing.mock.icephys import mock_VoltageClampSeries

mock_VoltageClampSeries()

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the 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 "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1595 (6c33889) into dev (31b887d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1595   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files          25       25           
  Lines        2510     2510           
  Branches      474      474           
=======================================
  Hits         2289     2289           
  Misses        140      140           
  Partials       81       81           

Flag	Coverage Δ
integration	70.35% <ø> (ø)
unit	84.42% <ø> (ø)

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[rly]

I did not know this was a thing in Python!

[bendichter]

pretty cool, huh?