input: Write a function to generate a Python class to represent a 3-dimensional vector. The class should have an initializer to set the x, y, and z components and a method to calculate the length of the vector. If the class is correctly implemented, it should pass the following test cases:
  
  assert Vector3D(1, 2, 2).length() == 3.0
  assert Vector3D(3, 4, 0).length() == 5.0
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(6, 8, 0).length() == 10.0
  assert Vector3D(1, 1, 1).length() == 3.0**0.5
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