More hints for Oblig 2

I have revised Oblig 2 slightly: added a hint for 2.13 and I would prefer that you test your code on the spline with coefficients  c=(-1,1,-1,1,-1,1,-1,1,-1), not on a single B-spline. If you already have tested you code on another spline / B-spline I will not require you to correct this.