Natural sort in Schedule.print_slacks when sorting on ids
Now bfly10 comes before bfly2.
See https://stackoverflow.com/questions/4836710/is-there-a-built-in-function-for-string-natural-sort
Now bfly10 comes before bfly2.
See https://stackoverflow.com/questions/4836710/is-there-a-built-in-function-for-string-natural-sort