new NCBI Nucleotide web translator

Using the Entrez Eutils JSON API. This is a translator for datasets, and therefore has some limitations, e.g. apparently no author information.
It's a bit late to be checking `items` here, since the call to `Zotero.selectItems` would have already failed (which is OK. The error it throws is quite informative, but given proper detection in `detectWeb` should never happen)