input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have a constructor to initialize the vector components (x, y, z) and a method to calculate the length of the vector.
  assert Vector3D(3, 4, 0).length() == 5.0
  assert Vector3D(1, 1, 1).length() == (3 ** 0.5)
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(2, 2, 1).length() == (9 ** 0.5)
  assert Vector3D(5, 12, 0).length() == 13.0
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
  
      def length(self):
          return (self.x ** 2 + self.y ** 2 + self.z ** 2) ** 0.5