Feature/litteratursiden reviews

Well, this function only ever receives numbers from Ting, where they might be formatted using spaces or dashes. We could add logic for checking that it is indeed a number consisting of 13 characters, but at this point it would be to overengineer things. We will probably never have a case where we wish to see if a string is indeed an ISBN-number. We need it to convert a string to an ISBN-13 number we can use. The case returning false is only present, as we might accidentally feed this function an ISBN-10 number instead.