input: Write a function to determine the sum of all elements in a given binary tree. If the given tree is empty then return 0.
  assert sum_tree(None)==0
  assert sum_tree(Node(1, Node(2, Node(3), Node(4)), Node(5, Node(6), Node(7))))==28
  assert sum_tree(Node(10, Node(5, None, Node(2)), Node(15, None, Node(7))))==49
  assert sum_tree(Node(5, None, None))==5
  assert sum_tree(Node(0, Node(0, None, None), Node(0, None, None)))==0
output: def sum_tree(root):
      if root is None:
          return 0
      return root.value + sum_tree(root.left) + sum_tree(root.right)