Fix up scratch API

[rly]

Motivation

Fix #1307, add missing unit tests, fix incorrect docval, clean up warnings and errors.

ScratchData requires notes, so set the default to empty string. A warning is raised in add_scratch if the notes argument is None or empty string. A warning is also raised in add_scratch if a pandas.DataFrame is provided and the table_description argument (required for DynamicTable) is None or empty string.

@bendichter I remember you saying something about empty strings being problematic. Could you elaborate?

Checklist

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[oruebel]

A key problem with empty string is that it is basically just side-stepping a required field by leaving it blank. I think requiring that users provide a description for the data they put into scratch is not much of a burden and at least provides some minimal documentation what those fields are about.

[rly]

This PR is ready to go. Previously the default values for 'notes' and 'table_description' were empty string (both are required in the schema). I changed these arguments to be required, which is a breaking change. This will be pushed in the next version, PyNWB 2.0.0, which will include other breaking changes (e.g. TimeSeries.data will be required).

[ajtritt]

I agree with Oliver.