review fix of exercise 5.8 polygon

[bjornregnell]

@EliasAAradsson Can you please review my fixes here:
6b3d666
and especially confirm that the code runs OK without errors or exceptions?

[bjornregnell]

added some more commits with minor fixes:
https://github.com/lunduniversity/introprog/commits/master/compendium/modules/w05-classes-exercise.tex

[bjornregnell]