Add Lua method to check whether a scenegraphnode is present in the scene

Make use of the function to guard mark_interesting_nodes against a nonexisting scenegraph node

scripts/scene_helper.lua

@@ -1,5 +1,7 @@
 mark_interesting_nodes = function(nodes)
     for _, n in pairs(nodes) do
-        openspace.addTag(n, "GUI.Interesting")
+        if openspace.hasSceneGraphNode(n) then
+            openspace.addTag(n, "GUI.Interesting")
+        end
     end
 end

src/scene/scene.cpp

@@ -416,6 +416,13 @@ scripting::LuaLibrary Scene::luaLibrary() {
                 &luascriptfunctions::removeSceneGraphNode,
                 "string",
                 "Removes the SceneGraphNode identified by name"
+            },
+            {
+                "hasSceneGraphNode",
+                &luascriptfunctions::hasSceneGraphNode,
+                "string",
+                "Checks whether the specifies SceneGraphNode is present in the current "
+                "scene"
             }
         }
     };

src/scene/scene_lua.inl

@@ -390,4 +390,16 @@ int removeSceneGraphNode(lua_State* L) {
     return 1;
 }
 
+
+int hasSceneGraphNode(lua_State* L) {
+    int nArguments = lua_gettop(L);
+    SCRIPT_CHECK_ARGUMENTS("removeSceneGraphNode", L, 1, nArguments);
+
+    std::string nodeName = luaL_checkstring(L, -1);
+    SceneGraphNode* node = OsEng.renderEngine().scene()->sceneGraphNode(nodeName);
+
+    lua_pushboolean(L, node != nullptr);
+    return 1;
+}
+
 }  // namespace openspace::luascriptfunctions