Currently, (in the new lib/interpolated.c) Gnuastro does a non-parametric interpolation (median value of a given number of nearest non-blank values). Non-parametric interpolation is very useful to avoid bad fits and impose the minimal assumptions on the result. So currently it is the preferred/only way in Gnuastro.

But parametric interpolations can also be useful in some contexts and GSL provides a good collection of them.

So it would be good if we could write wrappers for those functions in `lib/interpolate.c' to make it easier to use them with Gnuastro's `gal_data_t'.