[low priority] add computeDerivative method
When we know that the derivative of a curve will be used a lot, it may be a lot less computationally expensive to compute a new curve representing the derivative of a curve instead of calling derivate() several times. Ideally, once this curve is computed one can store it and simply call it's operator () when calling the derivate() method.
It was done in hpp-spline for bezier curves already, it will be really easy to implement it for polynomial too.