Fix exercise three from first set thanks to an observation of Giovanni Pinto.
Fix the drawCurve function in order to exclude the very last threshold 1
and stop a little before (wrt machine precision eps for Float64).

Fix exercise two for the first curve in order to get the maximum not for
the entire vector but for a prefix that ends at b, ie discarding the last
(k-1) elements.
Move exercises in a dedicated julia source file.