Adding of weighting functions support

[Дима Стрижевич]

Hi! It would be nice to include the standard weighting functions in the
next release of the GSL (like these
https://en.wikipedia.org/wiki/List_of_window_functions).
It is strange that this is still not there, because these functions are
constantly required both in statistics and for digital signal processing
(frequency analysis, digital filter synthesis, etc.)

Regards, Dzmirty.