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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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