Verify notebook results

[Zarquan]

In recent tests there was an issue with matplotlib and none of the tests generated any plots. However, all the tests passed.
We need a mechanism to verify the results of a test are as expected.

For the generated images, perhaps we can checksum the image contents and store it as an expected value in the metadata for a test.

[Zarquan]

Bumped this to high priority because the tests said all was OK when it wasn't.

[stvoutsin]

Ok thanks for reporting, will look into this