AStar: add a method to PathIterable to return total and current path length

And add javadocs.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
AStar: add an implementation that finds path along high intensities

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>