input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an initializer to set the x, y, and z coordinates, and a method to calculate the length of the vector.
  assert Vector3D(0, 3, 4).length()==5.0
  assert Vector3D(1, 0, 0).length()==1.0
  assert Vector3D(1, 1, 1).length()==3**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