input: Write a function to calculate the sum of all elements in a binary tree. If the tree is empty, the sum should be 0.
  assert sum_tree(None)==0
  assert sum_tree(Tree(1, Tree(2, Tree(4), Tree(5)), Tree(3)))==15
  assert sum_tree(Tree(10, Tree(5, Tree(2), Tree(7)), Tree(15, None, Tree(20))))==60
  assert sum_tree(Tree(8, Tree(4, Tree(2), Tree(6)), Tree(12, Tree(10), Tree(14))))==60
output: def sum_tree(root):
      if root is None:
          return 0
      return root.value + sum_tree(root.left) + sum_tree(root.right)