cleanup
GraphPairExtension: added SinkItemOf, ContainsSinkItemOf, SourceItemOf
FilteredGraph: implemented CopyTo, Count
DigidocSampleFactory: more pages, use SchemaThingGraph
SampleGraphPairFactory: use GraphItemTransformer<TSinkItem, TSourceItem> instead of  GraphItemTransformer<TSourceItem, TSinkItem,>
SceneMeshTest made generic, extracted EntitySceneMeshTest as SceneMeshTest<IGraphEntity>
VisualThingSceneMeshTest : SceneMeshTest<IThing> introduced
some improvements with VisualThingGraph-Handling