Expose pbg-ld data through Web API

[arnikz]

A prototype implementation based on example queries can be found here.

• [x] write SPARQL queries for grlc
• [x] look at some domain-centric examples:
  • BrAPI
  • Gramene API (v59)
  • Ensembl API
  • OpenPHACTS API
  • neXtProt API
  • EGA API