Add allow_extra=True to NWBFile.add_x_column

[rly]

Motivation

Fix #1860. TODO: add roundtrip test

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.95%. Comparing base (751983b) to head (9c3c171).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1861   +/-   ##
=======================================
  Coverage   91.95%   91.95%           
=======================================
  Files          27       27           
  Lines        2661     2661           
  Branches      699      699           
=======================================
  Hits         2447     2447           
  Misses        142      142           
  Partials       72       72           

Flag	Coverage Δ
integration	71.06% <100.00%> (+0.26%)	⬆️
unit	84.14% <100.00%> (ø)

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[rly]

I also added a function to help make test extensions and load them into type maps. That was significantly more work than fixing the bug in the first place, but now we can be sure that the custom column class with extra attributes can be roundtripped and exported correctly.