input: Write a python function to calculate the height of a binary tree.
  assert get_tree_height(BinaryTree(1)) == 0
  assert get_tree_height(BinaryTree(1).insert_left(2).insert_right(3)) == 1
  assert get_tree_height(BinaryTree(1).insert_left(2).insert_right(3).get_left_child().insert_left(4).insert_right(5)) == 2
output: class BinaryTree:
      def __init__(self, root_obj):
          self.key = root_obj
          self.left_child = None
          self.right_child = None
  
      def insert_left(self, new_node):
          if self.left_child is None:
              self.left_child = BinaryTree(new_node)
          else:
              t = BinaryTree(new_node)
              t.left_child = self.left_child
              self.left_child = t
          return self
  
      def insert_right(self, new_node):
          if self.right_child is None:
              self.right_child = BinaryTree(new_node)
          else:
              t = BinaryTree(new_node)
              t.right_child = self.right_child
              self.right_child = t
          return self
  
      def get_left_child(self):
          return self.left_child
  
      def get_right_child(self):
          return self.right_child
  
  def get_tree_height(tree):
      if tree is None:
          return -1
      left_height = get_tree_height(tree.get_left_child())
      right_height = get_tree_height(tree.get_right_child())
      return max(left_height, right_height) + 1