Makes Notebooks a part of testing (Issue 74)

[joshcombes]

This PR adds notebooks to our testing procedure using nbval. See issue #74 .

The PR includes:

• some small fixes to notebooks that were not working. (The only remaining notebook that does not work is the process tomography, it is documented here Tomography #94 .)
• additions to conftest.py to enable skipping slow tests
• additions to requirements.txt of nbval
• a new test file test_example_notebooks.py (notebooks which are longer than 30 seconds are skipped). The test passes if all the cells in a notebook can execute.

[joshcombes]

Thanks to @karalekas fix the code should work locally and remotely.

[joshcombes]

This is weird. The tests pass on gitlab but the semaphoreci build fails. @karalekas thoughts?

[karalekas]

@joshcombes Two things. (1) Currently the GitLab tests skip the ones that use qvm/quilc (which is a lot of them, so I need to fix that), so there is certainly a chance the GL and Semaphore outputs could be different. (2) Are you able to see the output of the tests on Semaphore? The failure is due to SCS, which is finicky (see #61) and therefore we decided to disable unit tests involving SCS. Take a look at the associated PR to see how we check for the SKIP_SCS environment variable and skip tests involving SCS, and if you want you can try to implement that for the distance_measures notebook tests that are failing (I'm happy to do it as well, but might be a good idea to spread the knowledge). Lmk!

[joshcombes]

@karalekas the Semaphore thing is hard to decode. In this case it was an easy fix using an nbval option.

[joshcombes]

[kylegulshen]

LGTM but we should either include the process tomography test or make an issue if the notebook isn't working.