Request for change: use Fortran 2008 standard mathematical functions
The Fortran 2008 standard introduces many mathematical functions: https://fortranwiki.org/fortran/show/Fortran+2008
These functions are for the moment defined internally in Octopus and I am suggesting to remove the internal code and use instead the following intrisinc functions:
Bessel functions: bessel_j0, bessel_j1, bessel_jn, bessel_y0, bessel_y1, and bessel_yn.
Error function: erf, erfc, and erfc_scaled.
Gamma function: gamma and log_gamma.
This reduces the dependence to GSL and remove the corresponding interfaces.
@martin.lueders @AlexBuccheri @hmenke @sohlmann @micael.oliveira Let me know if you agree with this. I am happy to do the changes if you agree.