input: Write a function to calculate the sum of all elements in a given binary tree. If the tree is empty (i.e., the root is None), return 0.
  assert sum_tree(None)==0
  assert sum_tree(Node(1, Node(2), Node(3)))==6
  assert sum_tree(Node(1, Node(2, Node(4), Node(5)), Node(3, Node(6), Node(7))))==28
  assert sum_tree(Node(5, Node(10, Node(20), Node(30)), Node(15, Node(25), Node(35))))==140
output: def sum_tree(root):
      if root is None:
          return 0
      return root.value + sum_tree(root.left) + sum_tree(root.right)