Add back DEP reference to citations.bib

[rickecon]

@jdebacker. I only noticed after I merged PR #941, that the change to citations.bib in which you removed the BibTeX reference to DEP:2019 that we no longer have a reference to that paper in the "Citations and Use Cases" chapter of the documentation. I don't know why there was an error, because the two references had different tags (DEP:2019 vs DeBackerEtAl:2019). But we should add that reference back to the "Citations and Use Cases" chapter.

[jdebacker]

@rickecon Having the same citation (even with a different tag) didn't cause an error, but did raise a warning. In trying to resolve Issue #747 and remove all warnings, I excluded the DEO paper from the "Citations and Use Cases" list since if it's only in one list, I think it's more appropriate in the "References" list since we cite it regarding tax functional form.

But I agree that it'd be best to have in both.