input: Write a python function to calculate BMI (Body Mass Index) given height in meters and weight in kilograms.
  assert calculate_bmi(1.75, 80) == 26.122448979591837
  assert calculate_bmi(1.8, 70) == 21.604938271604937
  assert calculate_bmi(1.6, 60) == 23.4375
output: def calculate_bmi(height, weight):
      return weight / (height ** 2)