input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer to set the x, y, and z coordinates, and a method to return a string representation of the vector in the form "(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(-1, -2, -3)) == "(-1, -2, -3)"
  assert str(Vector3D(0, 0, 0)) == "(0, 0, 0)"
  assert str(Vector3D(1.5, 2.5, 3.5)) == "(1.5, 2.5, 3.5)"
  assert str(Vector3D(-1.5, -2.5, -3.5)) == "(-1.5, -2.5, -3.5)"
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) + ")"