Added a delta time member variable to openspace engine and changed how dt is calculated.

it is now averaged over two frames instead of being just dt between two frames. This gives more stability

include/openspace/engine/openspaceengine.h

@@ -98,6 +98,7 @@ private:
 	ghoul::cmdparser::CommandlineParser _commandlineParser;
 	LuaConsole _console;
 	GUI _gui;
+	double _dt;
 
 	SyncBuffer* _syncBuffer;