Add __citation__ PEP compatibility

[jhale]

As discussed at NumFOCUS summit.

https://github.com/adrn/CitationPEP

[chrisrichardson]

What should go in the citation? The FEniCS Book (which is gradually becoming obsolete)? Or should we push to zenodo, get a doi, and use that as the citation? (which is a bit chicken and egg) - can we get a DOI from zenodo before packing up the repo?

[jhale]

The FEniCS Book is obsolete, apart from describing the academic goals of the project. On the other hand, it does pickup citations for some people :)

I know that Figshare will issue DOIs prior to making an item public. I find Zenodo a bit complex, but I'm sure it will do the same.

[wence-]

You can pre-reserve a zenodo DOI for an upload before finalising the contents.