make PatchClampSeries gain optional to match the schema

[stephprince]

Motivation

Fix #1971. PatchClampSeries.gain is an optional dataset in the schema but required by pynwb. This PR makes it optional in pynwb as well.

How to test the behavior?

Follow the icephys tutorial but remove gain as an argument from the 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 ruff check . && codespell 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

All modified and coverable lines are covered by tests ✅

Project coverage is 91.87%. Comparing base (3c0dbfa) to head (d6de01e).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1975   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files          27       27           
  Lines        2697     2697           
  Branches      704      704           
=======================================
  Hits         2478     2478           
  Misses        145      145           
  Partials       74       74           

Flag	Coverage Δ
integration	72.71% <100.00%> (ø)
unit	83.27% <100.00%> (ø)

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