Fix canonization of the alphatree and watershed hierarchy

The canonization was not canonizing the nodes at level 0 in the alphatree and watershed hierarchy (#82 (closed)). This problem is solved.

• Fixing the canonization issue #82 (closed)
• Correction of the tests

Futhermore, some minor changes:

• Correct the documentation issue for next (#85 (closed))
• Add a propagation option to the component tree attribute function computations