Tracking

Commercial message: improve the processing rate by 1000% !

Technical message: integrated opencv's tracking feature into Chilitags. A call to the "track()" method can replace one to find(). It is a lot faster (4ms with one tag on a 1600x1200px image with a laptop processor, versus ~35ms), but can not detect new or disappeared tags.

Tracking can also improve a little the detection, for tags that are moving to fast for find(). Such improvement can be toggled with setFindAndTrack()

It can be used in several ways:
* speed up detection when expected tags are known: call track() when the expected tags are found, find() otherwise
* improve responsiveness (at the expense of accuracy) by calling find() after every n calls to track()
* improve responsiveness by calling find() in a thread, and track in another (not tested yet)
* speedup detection by calling find() only in areas of the input image that have changed significantly, and track otherwise 

Short comings:
* Not propagated into Chilitags3D yet
* This includes a sample, but no test

This PR is based on the performance tuning PR: https://github.com/chili-epfl/chilitags/pull/37