Add notebooks to GitHub actions

[mj-gomes]

Hi,

As discussed in the last meeting, we need to put the notebooks as part of the CI tests. This PR creates a new separate workflow for notebook tests, using nbmake. I added it as a separate workflow because it can be slow, since notebooks often need large enough simulations to be able to show maps etc. This way, it runs in parallel with the other tests.

For commits where we want to skip the notebook tests, we can add [skipnb] or [skip nb] to the commit name.

I also fixed somes bugs in the notebooks that were leading to fails due to some lack of updates on the notebooks with the recent changes in lbs.

Please note:

1. we should probably now add assertions to each notebook; right now the tests only see if the notebooks yield some error or not, therefore not checking their physical correctness.
2. the changes done in hwp_harmonics in this PR should be temporary. I realized when fixing the errors in the notebooks that there is something weird on that module when using galactic coordinates instead of ecliptic. I am on it. What I did now made the notebooks work but also made some of the hwp tests fail. I hope I find the reason to this in the next days and fix this.

After these two things, I believe this PR will be ready to go. For now, I leave it as a draft.

[ggalloni]

Hi @mj-gomes, thanks for initiating this!

As a matter of fact, these days I did something similar to test the notebooks on another repo, so I might be able to help with this 👍