turns path into an absolute path and checks that it exists, then
        appends it to the args using the given flag (or none).

def check_input(self, flag, path):
        """
        turns path into an absolute path and checks that it exists, then
        appends it to the args using the given flag (or none).
        """
        path = os.path.abspath(path)
        if os.path.exists(path):
            if flag:
                self.args.append(flag)
            self.args.append(path)
        else:
            utils.die("input file for flag '%s' does not exists:\n  %s" % (
                      flag, path))