ConvexBranchesDecomposition: add a utility to build branch graph.

And put some more javadocs.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Exploit the layout in branches for "complicated" tracks.

Tracks that have merging points always were a mess to arrange in
TrackScheme. This makes it slightly better.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>