Some code comments calling for BSpline modifications

Hi Sacha @sacha.duverger

Please have a look at these comments in 6baa2534 about BSpline functions expressions. Outside of the duplicated code issue I mentioned in !24 (merged) (which is even more important now we actually modify that piece of code), I have doubts about correctness of the expressions (probably I forgot where the initial ones came from ? From your Python script ? With which knot values ?), if one compares with the "boundary modified" choices of e.g. Nguyen2023