Develop and execute SPARQL competency questions as test suite and jupyter notebooks

[markdoerr]

Collection of SPARQL base competency questions leading to a pytest / rdflib based test suite and jupyter notebooks illustrating how to query some facts from our examples.

This issue corresponds to
dcat-ap-plus/issues/57

[markdoerr]

I created a new branch for these developments:
feature/122-sparql-competency-questions-testsuite