Implemented ObjectGraphNode::operator<().
ObjecGraph now stores nodes in a set.
ObjectGraph::findNode() now uses set functionality.
ObjectGraph::insertNewNode() now checks if node already exists and returns pointer to existing nodes.

If you add two nodes with the same coordinates, only one node is inserted. Adding the second node will return a pointer to the first inserted node.
Made less-operators const.
ObjectGraph saves nodes inside a map and adds new edges to nodes.
Renamed ObjectGraph insert methods.