Small trees/tables fail

[ElDeveloper]

Specifically this test from skbio's test suite fails:

    def test_weighted_minimal_trees(self):
        # two tips
        tree = TreeNode.read(StringIO('(OTU1:0.25, OTU2:0.25)root;'))
        actual = self.weighted_unifrac([1, 0], [0, 0], ['OTU1', 'OTU2'], tree)
        expected = 0.25
        self.assertEqual(actual, expected)