In order to have a complete mapping of the psinc basis into the G-space,
 we need to keep the entire G-bx, and not to trunctate it within the
 kinetic energy cutoff !!

 All the nice properties of the psinc's are based on this full mapping,
 if we truncate the orthogonality and all that vanish.
 
 I think we need to define a grid spacing such that the corresponding
 G-box is CONTAINED within the sphere of the G-vectors smaller than
 the cutoff. In practice the calculation HAS TO BE RUN with a cutoff
 smaller than for the wavefunctions. I think this is still fine, since
 calculations of epsilon are usually done with 5 or 10 Ry.