Improvements for next iteration of tutorial notebooks

[rly]

New users are using the tutorial jupyter notebooks as a template for their data conversion scripts. This is problematic because the tutorial notebooks do not cover all the various use cases, e.g., adding raw data from an ecephys experiment.

The tutorial should

1. point users to the PyNWB API documentation
2. demonstrate use of the validator since it is possible to use PyNWB to create invalid files
3. explain common errors such as re-creating a table column that already exists or attempting to set nwbfile.subject twice and how to resolve them

It would also be helpful to provide users with a template conversion script/notebook that works on a dummy data file that is as comprehensive as possible, using every optional field, and with comments noting how data can be stored in other ways.

This was also suggested at Cosyne.