input: Write a function to create a Python class that represents a 3-dimensional vector with an initialization method to set the vector's coordinates and a method to calculate its length.
  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(3, 4, 0).length() == 5
  assert Vector3D(0, 0, 0).length() == 0
  assert Vector3D(1, 1, 1).length() == (3**0.5)
  assert Vector3D(2, 2, 2).length() == (12**0.5)
  assert Vector3D(6, 8, 0).length() == 10
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