added Sparqlify dependency
added rr vocabulary URI constants
added sample SML file and type mappings
added SML to R2RML converter from old repo

- took the code from https://github.com/AKSW/Sparqlify-Extensions and
  made it work with a current Sparqlify version
- just tested on a small example view definition file --> there may be
  still errors
- I can remember that I once added the functionality to merge triples
  maps if they refer to the same logical table and subject map but I did
  not find the corresponding code in the given repo's master branch