New draft for the flow-shop chapter.

Latest changes according to feedback from @mdibernardo and a completed chapter draft.
I think changing the variable naming/labeling here would help, too. I found it hard to keep this state consistent in my head, since there are a lot of numbers flying around. For example, job t-1-1 and t-1-2 take 1 and 2 minutes, which primes my brain to think that the task length is being represented in the job label. Then, t-2-1 and t-2-2 take 2 and 1 minutes, which makes me stumble a little bit cognitively, and I have to go back and check that I read everything correctly.