Precompute $\tilde \psi$

The precomputation of the values $\tilde\psi\left({\ensuremath{\boldsymbol{x}}}_j - \ensuremath{\boldsymbol{n}}^{-1}\odot \ensuremath{\boldsymbol{l}}\right)$ , i.e. the entries of the matrix $\ensuremath{\boldsymbol{B}}$ , depends on the choice for my_plan.x. If one of the proposed precomputation strategies (PRE_LIN_PSI, PRE_FG_PSI, PRE_PSI, or PRE_FULL_PSI) is used, one has to call the appropriate precomputation routine AFTER setting the nodes. For simpler usage this has been summarised by

if(my_plan.nfft_flags & PRE_ONE_PSI) nfft_precompute_one_psi(&my_plan);

Note furthermore, that

1. FG_PSI and PRE_FG_PSI rely on the Gaussian window function,
2. PRE_LIN_PSI is actual node independent and in this case the precomputation can be done before setting nodes,
3. PRE_FULL_PSI asks for quite a bit of memory.