Ported from R.
Contains:
double besselI(double x, double alpha, Flag!"ExponentiallyScaled" expFlag = Flag!"ExponentiallyScaled".no);
double besselJ(double x, double alpha, Flag!"ExponentiallyScaled" expFlag = Flag!"ExponentiallyScaled".no);
double besselK(double x, double alpha, Flag!"ExponentiallyScaled" expFlag = Flag!"ExponentiallyScaled".no);
double besselY(double x, double alpha, Flag!"ExponentiallyScaled" expFlag = Flag!"ExponentiallyScaled".no);