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