Fix ElectricalSeries->electrodes dataset write and add tests

[stephprince]

The ElectricalSeries->electrodes dataset was being written incorrectly due to an incorrect data type. I updated the data type and added tests for write/read of this ElectricalSeries->electrodes dataset to avoid this issue in the future.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.98%. Comparing base (d234fa0) to head (5d0986e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   93.93%   93.98%   +0.04%     
==========================================
  Files          64       64              
  Lines        4915     4952      +37     
  Branches      272      273       +1     
==========================================
+ Hits         4617     4654      +37     
  Misses        288      288              
  Partials       10       10              

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