Write a function, format_number_list, whose argument is a list of
integers.  It then returns a string which represents the input list in
compact, human-readable form.

For example,

         format_number_list(1, 2, 4, 5, 6, 7, 9, 13, 24, 25, 26, 27)

will return

        "1-2, 4-7, 9, 13, 24-27"

Also write a function, 'expand_number_list', which does the conversion
in the opposite direction, so that

        expand_number_list("1-2, 4-7, 9, 13, 24-27")

will return

        (1, 2, 4, 5, 6, 7, 9, 13, 24, 25, 26, 27)


(Thanks to Mark Hickman for suggesting this quiz problem.)