Feature/anchor and aim (#799)

Introduce the ability to navigate using an anchor and aim.
Example use: Set spacecraft as anchor and planet as aim to always look down at a planet, while followin the spacecraft in its orbit.

src/query/query.cpp

@@ -37,6 +37,9 @@ Scene* sceneGraph() {
 
 SceneGraphNode* sceneGraphNode(const std::string& name) {
     const Scene* graph = sceneGraph();
+    if (!graph) {
+        return nullptr;
+    }
     return graph->sceneGraphNode(name);
 }