input: Write a function to generate a Python class named Vector3D to represent a 3-dimensional vector with an initializer method to set the x, y, and z coordinates and a method to calculate the length of the vector.
  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
  
  assert Vector3D(1, 2, 2).length()==3.0
  assert Vector3D(3, 0, 4).length()==5.0
  assert Vector3D(0, 0, 0).length()==0.0
  assert Vector3D(5, 12, 0).length()==13.0
  assert Vector3D(8, 15, 0).length()==17.0
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