Big refactoring

The main goal of this refactoring was to get rid of the "legacy", e.g. the Pipeables, the Quad class, and the ugly singleton (Registrar) which was at the base of the Chilitag class (RIP).
It followed a general simplification of the code and of the interface (the pose estimation code took the biggest OCD hit), and a reorganisation of the file structure.
Along the way, more tests were added, as well as some tools (a graphical display of the pose estimation, and a utility to save the last image before a crash)