Precompute $\tilde \psi$

The precomputation of the values $\tilde\psi\left({\mbox{\boldmath {${x}$}}}_j - \mbox{\boldmath {${n}$}}^{-1}\odot \mbox{\boldmath {${l}$}}\right)$ depends on the choice for my_plan.x. If and only if the nfft-flag PRE_PSI is set, the subroutine nfft_precompute_psi(&my_plan) has to be called, i.e.

if(my_plan.nfft_flags & PRE_PSI) nfft_precompute_psi(&my_plan);