Add a + sign to the merge option to allow more than 2 rdf files to be merged

[labra]

It would be nice to be able to merge 3 or more RDF inputs...

[ulitol97]

I believe this is fairly doable:

• In the client, we currently support storing the user's RDF inputs in the browser's local storage. Several inputs are already admitted and handled correctly (see Data Merging tool, for instance), so the client's codebase is ready.
• In the API, more testing is required, but as long as the data sent is specified as COMPOUND it will try to merge all the inputs received before processing any data.

As long as we prepare the client's UI to handle this, and with enough testing, this functionality could be applicable to all tools handling data (not just merge). Therefore the user could upload several pieces of data that the server would merge transparently.