Don't create dummy nodes (node with a parent and just a child) in H0
Tree. That save lot of computation time.
We can use the attribute _phy instead of phy property of the node to
save time because we perform a lot of access to phy property while
computing H0Tree
When we use union() in the H0Tree algorithm we know exatly what we are
doing and we can save lot of time by use _union() that do the same thing
but without any checking
Faster is_leaf and children properties computation in order to speed up
the SF algorithm

SF_FACTOR to profile SF computation
Better use attrgetter instead of lambda function (simple to read) but
that dosn't give any advantage from a optimization point of view.
Non recursive implementation of get_min() and two useful property...
test will come soon.