input: Write a function to calculate the volume of a cone.
  assert cone_volume(5,12)==314.15926535897927
  assert cone_volume(10,15)==1570.7963267948965
  assert cone_volume(19,17)==6426.651371693521
output: def cone_volume(radius, height):
      import math
      return (1/3) * math.pi * radius**2 * height