Force all Renderables to declare an 'isReady' function to circumvent assertion triggers when a crucial element of initialization fails

Fix velocity in RenderableStars

include/openspace/rendering/model/renderablemodel.h

@@ -49,6 +49,8 @@ public:
     bool initialize() override;
     bool deinitialize() override;
 
+	bool isReady() const override;
+
 	void render(const RenderData& data) override;
     void update(const UpdateData& data) override;