input: Write a function to create a Python class that represents a 3-dimensional vector, allowing for initialization with three coordinates (x, y, z) and providing a method to calculate the length of the vector.
  assert Vector3D(3, 4, 0).length() == 5.0
  assert Vector3D(1, 1, 1).length() == (3**0.5)
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(5, 0, 0).length() == 5.0
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