rdflib should be removed from all scripts

[rchateauneu]

All references to rdflib RDF package are now exclusively done in the module lib_kbase.py ("kbase" stand fro knowledge base), which allows survol to use another representation of its graph (triplestore).

Except in two scripts, where rdflib is used only once in each:
survol/sources_types/rpm/init.py
survol/sources_types/Linux/installed_rpm_packages.py