Write out final RIS file

[LukasWallrich]

Once deduplication is done, a final RIS file should be written out. All sources that the record was found in should be included in the database field - possibly as a JSON, or simply comma separated?

[LukasWallrich]

Also allow download as a csv file to facilitate future analyses

[LukasWallrich]

I have now added export_csv() which allows users to write out a csv file for further analyses - please have a look, but that seems fine.

For export_ris(), we would like to use synthesisr, but that currently does not allow for the export of custom fields (a way to do that seems half-implemented - I raised an issue). For now, I overwrote one of their functions from within our package ... that is not particularly robust and will likely be rejected by CRAN, but it should be good enough for the MVP.

We still need to decide what fields to use. For now, the function defaults to DB for source, C7 for label and C8 for string (I thought the last two custom fields would be least likely to clash with other info, and Wikipedia at least only lists 8). According to synthesisr, C7 is used by some databases for an "article number" - but we can probably live with that?