Use individual notebook requirements

[jeffjennings]

• Removes the top-level requirements.txt in favor of individual requirements.txt specified in each folder in tutorials.

• Applies these individual requirements in the execute and convert commands of the Makefile. There, packages are force re-installed (not re-installed if the cache already contains the same version) for each group of notebooks (a group is the set of notebooks in each folder in tutorials).

• Adds a cell near the top of each notebook specifying the required packages to run the notebook (pulled from the relevant requirements.txt.

Note to reviewer: The diffs for the notebook files are showing changes to metadata, even though it seems like the pre-commit is supposed to strip that. The only change to each notebook file is the addition of the cell printing the required packages (and in a few cases the consolidation of markdown cells at the top of the notebook that constitute the preamble, to adhere to the 'Template intro guide' in case some search indexing is done that assumes the entire preamble is contained in the first cell).

Closes #601

• Check the box to confirm that you are familiar with the contributing guidelines and/or indicate (check the box) that you are familiar with our contributing workflow.
• Confirm that any contributed tutorials contain a complete Introduction which includes an Author list, Learning Goals, Keywords, Companion Content (if applicable), and a Summary.
• Check the box to confirm that you are familiar with the Astropy community code of conduct and you agree to follow the CoC.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[adrn]

Overall, I think this will be a good change for users!

I'd like to discuss one implication of this change, which is that in this new way of managing requirements, pinning dependencies now requires updating many files. It would be nice if we didn't have to keep track of this. But I can't think of a better way of managing that! Thoughts?

Relatedly, I can see a use case (at least for us as developers) for wanting to be able to install all dependencies in one go. The old top-level requirements.txt file provided this. Would it make sense to provide a makefile command that just loops through all of the tutorial directories and installs all requirements?

[jeffjennings]

Yes it's a good idea! Unfortunately there are a few cases where the version for the same requirement differs, making it hard to have a single, top-level set of requirements.

[adrn]

Thanks @jeffjennings -- LGTM!