input: Write a python function to create a BinaryTree, insert nodes to the left and right, and return the root value.
  assert create_tree().get_root_val() == 'a'
  assert create_tree().get_left_child().get_root_val() == 'b'
  assert create_tree().get_right_child().get_root_val() == 'c'
  
  def create_tree():
      tree = BinaryTree('a')
      tree.insert_left('b')
      tree.insert_right('c')
      return tree
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
  
      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
  
      def get_root_val(self):
          return self.key
  
      def get_left_child(self):
          return self.left_child
  
      def get_right_child(self):
          return self.right_child
  
  def create_tree():
      tree = BinaryTree('a')
      tree.insert_left('b')
      tree.insert_right('c')
      return tree