have some support in `sema.commons.clean` to verify appropriate usage of linked references

[mpo-vliz]

we recently had a case where sh:nodeKind got up to be sh:nodekind
through 'open ended' principles behind the semantic web -- this got just hidden and unnoticed for a long time

so maybe we can add some semweb-consistency check into py-sema?

idea is to have for a certain retrieved representation:

• listing all end-points (predicates, linked objects)
  --> this should be all in ?p or ?o that have no own predicates associated
• check those to be "findable on the web"

(of course the resulting report will depend on all involved to properly be making their resources dereferencable...)