ProcessorTracker compatible with preprocessing and other processors

This is a small issue/MR but somehow important. In ProcessorTrackerFeature and ProcessorTrackerLandmark, we consider that last_ptr_->getFeatureList() is the list of the tracked features (a.k.a the known features).

This is because an important assumption was made when developing: "this is the only processor working with the capture". Also, the design assumed that the features are "emplaced" in the time that they are tracked.

This has 2 consequences (that I can think of)

1. Unexpected behaviour if another processor is working with the same capture.
2. Need of workarounds if the processor detects all the features of the capture in preProcess().

I think that this would be easily addressed by adding FeatureBasePtrList known_features_last_ analogous to the known_features_incoming_. I think that it requires a small change in the code.

Then, the processor can emplace all detected features to the capture in preProcess() without need of creating auxiliar lists.

@jsola does it seems reasonable to you?