Add nbmake or equivalent pytest extension for notebook e2e tests

[aidanjgriffiths]

Add nbmake stage for notebook e2e tests to ensure notebooks don’t break during PRs.

https://github.com/treebeardtech/nbmake

[aidanjgriffiths]

Lol, ran pytest --nbmake tutorials/ and only 3 notebooks passed without failure😄 .

[tennlee]

Thanks for looking into this. For anyone else who might be reading along, I have been manually testing the notebooks carefully during the process of creating a release, so released versions should be stable and reliable. Having an effective CI/CD step would be really helpful, thank you.

[aidanjgriffiths]

No worries, its something I had to do about 3 years ago without a nice pytest extension so I'm partially interested on how well this works. I think most of the notebooks are failing due to not having the obs anf fcsts netcdfs cached locally.