Sync from github repository.

Many fixes including:
- #867248 Call-time pass-by-reference has been deprecated
- #925520 RDF file's prefix mapping overwrites user defined
- #926442 Provide link to import core vocabularies on installation (evoc)
- #924532 core API change: MergeQuery::update() does not exist anymore
- Improved tests for evoc
- new directory structure for the submodules