improve xkos-best-practice queries or use SHACL

[VladimirAlexiev]

https://github.com/FranckCo/Stamina/blob/master/doc/xkos-best-practice.md.
Such best practices are usually called Application Profiles. The best current way to describe them is with RDF Shapes (SHACL): https://www.w3.org/TR/shacl/, which specifies typical constraints and a way to express validation results. SHACL implementations include TQ SHACL API and AKSW RDFUnit.

Barring that, please simplify the queries. Eg instead of

SELECT ?s {
  ?s rdf:type skos:ConceptScheme .
  MINUS {
    SELECT ?s {
      ?s rdf:type skos:ConceptScheme .
      ?s skos:notation ?code .
    }
  }
}

use this:

SELECT * {
  ?s a skos:ConceptScheme
  FILTER NOT EXISTS {?s skos:notation ?code}
}