input: Write a python function to create a binomial tree and add left and right children.
  assert create_binomial_tree(1).data == 1
  assert create_binomial_tree(1).left == None
  assert create_binomial_tree(1).right == None
  assert (lambda tree: (tree.add_left_child(2), tree.left.data == 2))(create_binomial_tree(1))
  assert (lambda tree: (tree.add_right_child(3), tree.right.data == 3))(create_binomial_tree(1))
output: class BinomialTreeNode:
      def __init__(self, data):
          self.data = data
          self.left = None
          self.right = None
  
      def add_left_child(self, data):
          self.left = BinomialTreeNode(data)
          
      def add_right_child(self, data):
          self.right = BinomialTreeNode(data)
  
  def create_binomial_tree(data):
      return BinomialTreeNode(data)