Returns a leaf at the given index.
        :param index:
        :return: leaf (value) at index

def get_leaf(self, index):
        """
        Returns a leaf at the given index.
        :param index:
        :return: leaf (value) at index
        """
        leaf_level_index = len(self.tree['levels']) - 1
        if index < 0 or index > len(self.tree['levels'][leaf_level_index]) - 1:
            # index is out of bounds
            return None
        return self.tree['levels'][leaf_level_index][index]