input: Write a function that creates a Python class representing a 3D Vector. The class should have an initializer to set the x, y, and z coordinates, and a `__str__` method to return the vector in the format "(x, y, z)".
  
  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) + ")"
  
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(0, 0, 0))=="(0, 0, 0)"
  assert str(Vector3D(-1, 4, 2))=="(-1, 4, 2)"
output: 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) + ")"