input: Write a function that generates a Python class representing a 3D Vector with initial coordinates and a method to return the vector as a string 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, 5, 10))=="(-1, 5, 10)"
  assert str(Vector3D(3.14, 2.71, 0))=="(3.14, 2.71, 0)"
output: 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) + ")"