Problem loop-closure callback
We need to notify problem (from the processor side) that a loop has been closed.
This is the API for key-frame callback (for inspiration purposes)
void keyFrameCallback(FrameBasePtr _keyframe_ptr, //
ProcessorBasePtr _processor_ptr, //
const Scalar& _time_tolerance);
To be discussed: What should the problem do with this notification. We could raise a flag and then a method to get/clear the state of this flag.