add trials table to nwb file

[bendichter]

Motivation

The current intro tutorial creates a trials table but never adds it to the nwb file. This fixes the mistake

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.60%. Comparing base (c51099f) to head (1b7b657).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #599   +/-   ##
=======================================
  Coverage   90.60%   90.60%           
=======================================
  Files         106      106           
  Lines        4671     4671           
=======================================
  Hits         4232     4232           
  Misses        439      439           

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