SearchTerm: allow property negation
acdhOeaw\arche\lib\SearchTerm - allow property to be negated by prepending it with a acdhOeaw\arche\lib\SearchTerm::PROPERTY_NEGATE. When negated, the rdfs:resource search term type is enforced and the search term value is evaluated against the triple subject resource instead of the object resource. A canonical use case would be a search for a parent of a given resource.