yukawa-kernel

As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations.  So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning.  We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).