input: Write a function to calculate the square root of a user-defined positive number. Use the standard library to perform the calculation. If the input number is not positive, return an error message. If the input number is zero, return zero.
  assert sqrt_calculator(100)==10.0
  assert sqrt_calculator(25)==5.0
  assert sqrt_calculator(0)==0
  assert sqrt_calculator(-1)=='Error: Input must be a positive number.'
output: def sqrt_calculator(num):
      if num < 0:
          return 'Error: Input must be a positive number.'
      elif num == 0:
          return 0
      else:
          return num ** 0.5