Add cleanup function

[thatandromeda]

In the case of multi-author papers, we don't want to request the same paper from a bunch of authors if one has already submitted. Questions to think through include...

• If we get an import of a PaperID that we have already requested from a different author, check to see if it has been received (in Elements) and decide on the citation disposition accordingly
• What if we import a PaperID that we already know about but have not yet requested?
• What if we import multiple citations for the same PaperID in one import process?
• Do we try to stop people from composing multiple emails about the same paper in a single workflow?

[thatandromeda]

Current status: we reject PaperIDs that have already been requested; we are happy to import multiple unrequested versions from different authors; we don't try to remove the citation from other emails after the first. Also there's some technically challenging stuff here; see #38 .