Add citations to packages, models, and datasets used

[hdrake]

We currently provide url's to the documentation for many (but not all) packages, models, and datasets in the tutorials. Perhaps there is a better form of referencing these (e.g. linking to their DOI's if they have them?).

If we release a formal version of our 2020 PraCTES workshop notes via something like Zenodo, I know that we then get a DOI and can get cited – but does that also let us cite others that we depend on?

Thoughts @gaelforget?