Store Yake suggestions

[hekl]

I would like to see the suggestions that are generated by Yake stored somewhere. Especially when using the webserver or the API. Currently showing the suggestions only works through the command line per document. It would also be useful when these suggestions are available for a batch of documents, but I am not sure Yake supports that. Suggestions could be used to enhance the vocabulary you are using. Is there a big discrepancy between the vocabulary and the terms of your documents? This would be interesting to know. Are there new terms that do not figure in your vocabulary?

[osma]

Thanks for opening the issue. I see this as a more refined version of the older issue #224, also opened by you ;) But I decided that it's better to close the old issue and continue on this one, now that we actually have YAKE integrated with Annif. Returning the YAKE suggestions was something we had in mind also when integrating YAKE (see #439, #461), but at that time it was postponed for later.

I think the main outcomes of this should be:

1. the REST API should return the keyword suggestions as a separate list (alongside the subjects from the vocabulary)
2. the Web UI should display the keyword suggestions it receives from the API
3. The annif suggest CLI command should print the keyword suggestions
4. The annif index CLI command should save the keyword suggestions into the tsv files it produces