add dummy data for ecephys

[bendichter]

Motivation

fix #1452

I tried fixtures first but it was very difficult to get them to play nice with unitTests.

[bendichter]

I'm happy with the flexibility of this approach. I think this could help minimize a l of testing code across several repos, including PyNWB, NWB Widgets, and NWB Inspector.

Questions:

• How do people feel about this naming convention?
• Should we try to automate creation from docval?

[bendichter]

@rly and @oruebel this is ready for review