input: Write a function to calculate the square root of a given positive number. If the given number is not positive, return None.
  def calculate_square_root(n):
      if n <= 0:
          return None
      else:
          return math.sqrt(n)
          
  assert calculate_square_root(100)==10.0
  assert calculate_square_root(25)==5.0
  assert calculate_square_root(0)==None
  assert calculate_square_root(-1)==None
  assert calculate_square_root(9)==3.0
output: def calculate_square_root(n):
      if n <= 0:
          return None
      else:
          return math.sqrt(n)