input: Write a function to calculate the square root of a user-defined positive number using the math library. If the given number is negative, return a message indicating that the number must be positive. If the given number is zero, return zero.
  assert calculate_square_root(100)==10.0
  assert calculate_square_root(0)==0
  assert calculate_square_root(-1)=='The number must be positive.'
output: def calculate_square_root(number):
      if number < 0:
          return 'The number must be positive.'
      elif number == 0:
          return 0
      else:
          return math.sqrt(number)