Validation against SPARQL endpoint

New features

• Added command line options to validate against endpoints

• Repaired stackoverflow bug when validating wikidata endpoint with nodes that
  had a large volume of incoming arcs

TODOs

• ShEx: Complete UML generation for ShEx schemas

• ShEx: Complete semantic actions implementation

• SHACL: support using RDf4j (add SHACL paths to SRDF4j)

• SHACL: Check test-suite and create report

• ShEx: test-suite with shape maps and update report

• Shaclex: Conversion from ShEx to SHACL

• Shaclex: Conversion from SHACL to ShEx