input: Write a function to calculate the square root of a user-defined positive number. The function should return the square root value as a float. If the number is not positive, it should return None. 
  assert square_root(100)==10.0
  assert square_root(0)==None
  assert square_root(-1)==None
  assert square_root(25)==5.0
  assert square_root(16)==4.0
output: def square_root(number):
      if number < 0:
          return None
      return float(number ** 0.5)