import functionality for dcat records, using rdflib

[pvgenuchten]

This PR suggests the introduction of rdflib to parse dcat records in either json-ld, xml or turtle serialisation.

• if json and no context -> add a dcat-ap context
• let rdflib detect derialisation and parse document to graph
• parse the graph using element matching or sparql
• parse wkt-bbox using shapely

We can later extend this work to:

• add a serialisation parameter to the write method, to write out the knowledge graph in either json-ld, rdf+xml, or turtle (when converting from mcf to dcat
• use rdflib when importing schema.org (in various serialisations)
• parse Dublin Core records using this plugin

The PR introduces rdflib and shapely dependency, can also run conditionally, if import is available