Added ability to pass an instance of Graph to delete a graph in GraphHandler (incl. Test)

Added ability to pass an instance of Graph to retrieve graph properties

Added ability to pass an instance of Graph to remaining GraphHandler methods and added some test cases to GraphExtendedTest
Merge pull request #169 from olivermack/object-as-graph-argument

Added ability to pass an instance of Graph to delete a graph in GraphHandler (incl. Test)