input: Write a function to create a Python class that represents a 3-dimensional vector with attributes x, y, and z. The class should also include a method to calculate and return the length of the vector. If no attributes are provided, the class should initialize x, y, and z to 0.
  assert Vector3D(3, 4, 0).length()==5.0
  assert Vector3D(1, 0, 0).length()==1.0
  assert Vector3D(0, 0, 0).length()==0.0
  assert Vector3D().length()==0.0
output: class Vector3D:
      def __init__(self, x=0, y=0, z=0):
          self.x = x
          self.y = y
          self.z = z
  
      def length(self):
          return (self.x**2 + self.y**2 + self.z**2) ** 0.5