feat: Add ChEMBL and Mondo

[jsstevenson]

These are our two current problem children.

• ChEMBL: downloads via HTTP. I think FTP is probably faster, but I've been ~unable to maintain an FTP connection with the EMBL FTP server at the hospital, so I think it's easier to just go with HTTP, particularly since they don't do too many updates.
• Mondo: Includes some extra methods to support trying previous versions. The most recent Mondo release is unparseable by our OWL reader, and while I think they're going to make a change that resolves this, I don't think they view it as an urgent or critical fix (the problem is really with owlready2, not with their data output).

There's a little bit of repeated code here. I think it'd be better to hold off on consolidating methods until we've seen what it looks like to implement this for every source that we're interested in.