input: Write a function to calculate the square root of a user-defined positive number using the math module. If the number is positive, return the square root; otherwise, return a message stating that the number must be positive.
  assert calc_sqrt(100)==10.0
  assert calc_sqrt(25)==5.0
  assert calc_sqrt(0)=='Number must be positive'
  assert calc_sqrt(-10)=='Number must be positive'
output: def calc_sqrt(num):
      if num > 0:
          return math.sqrt(num)
      else:
          return 'Number must be positive'