GraphDB improvements + More Unit Tests

This MR contains the following improvements to the GraphDB component more unit tests

• We simplify the deleteClaims method implementation
• We add a validation of SdClaims, so they follow RDF specification, i.e., <URI, URI, (URI or String)>
• We use session.readTransaction in queryData method now for more clarity in the code
• Unit tests implemented for addClaims