- introduce variables ngX_o2 = ngX/2 where X = p,j,z
- replace all ngX/2 call by a call to ngX_o2
- compiler should optimize it anyway but can we trust it?

-kx is now 2D (ky dependant)
-kperp^2 array for computation reduction
-routines to update the grids

The perpendicular pressure effects are already added above
so this commented part is useles