Update of Oblig 3 text and added a bit of precode

There were a few updated to the text for Oblig 3 that was uploaded yesterday morning. Mainly, some leftovers from a previous text, that could result in misunderstandings.

Also, for Oblig 3 you should continue working from your code in Oblig 2 and extend this. To ease the start we will give you some help. Specifically, the REPL (read-evaluate-print-loop) that is needed to handle code.
You can find the precode.zip under Oblig 3, but I have also added a README that describes that have been added.