Add Tree.siblings(u) method

[jeromekelleher]

It is sometimes useful to have the list of siblings of a given node, and it would be handy to have the method to do it directly:

def siblings(self, u):
    sibs = []
    parent = self.parent(u)
    if parent != -1:
        sibs =  [v for v in self.children(parent) if u != v]
    return sibs

This should do the trick I think?