[SPARQL] Add a verification mechanism for variable instanciation in query modifiers

[MaillPierre]

Create a mechanism that will check that variables in the projection of SPARQL queries are actually used in the WHERE clause of the query, before the creation of the query AST.
Also apply to the variable in the GROUP BY, ORDER BY clause

CF:

• Projection: https://www.w3.org/TR/sparql11-query/#selectproject
• Solution Modifiers: https://www.w3.org/TR/sparql11-query/#solutionModifiers