input: Write a function to create a Python class named Vector3D that represents a 3-dimensional vector with attributes x, y, and z. The class should also have a method named length which returns the Euclidean distance of the vector.
  
  assert Vector3D(3, 4, 0).length()==5.0
  assert Vector3D(1, 2, 2).length()==3.0
  assert Vector3D(0, 0, 0).length()==0.0
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