input: Write a function named `create_vector_class` that generates a Python class representing a 3D Vector. The class should have an `__init__` method to initialize the vector with `x`, `y`, and `z` coordinates, and a `__str__` method to return the vector as a string in the format "(x, y, z)".
  
  assert str(create_vector_class())==
      "class Vector3D:\n    def __init__(self, x, y, z):\n        self.x = x\n        self.y = y\n        self.z = z\n    \n    def __str__(self):\n        return \"(\" + str(self.x) + \", \" + str(self.y) + \", \" + str(self.z) + \")\""
output: def create_vector_class():
      class Vector3D:
          def __init__(self, x, y, z):
              self.x = x
              self.y = y
              self.z = z
          
          def __str__(self):
              return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
      return Vector3D