Tree.num_samples() inefficient in common case

[jeromekelleher]

The current implementation is

 def num_samples(self, u=None):
        roots = [u]
        if u is None:
            roots = self.roots
        return sum(self._ll_sparse_tree.get_num_samples(u) for u in roots)

This is elegant, but the sum and iterator can make tight loops that call num_samples take significantly longer than calling _ll_sparse_tree.get_num_samples(u).

Separate into two different code paths.