neutral_vz_advection

calculate the advection speed in the vz-direction at each grid point

update the advection speed (for the neutral distribution function) in the z-velocity coordinate for the case where density, flow and pressure are evolved independently from the pdf; in this case, the parallel velocity coordinate is the normalized peculiar velocity wpahat = (vpa - upar)/vth

update the advection speed in the parallel velocity coordinate for the case where density and pressure are evolved independently from the pdf; in this case, the parallel velocity coordinate is the normalized velocity vpahat = vpa/vth

update the advection speed in the parallel velocity coordinate for the case where density and flow are evolved independently from the pdf; in this case, the parallel velocity coordinate is the peculiar velocity wpa = vpa-upar