added p_div(v) term to half-update on VL
added p_div(v) term to half-update on VL

cholla-hydro/cholla,cholla-hydro/cholla,cholla-hydro/cholla,cholla-hydro/cholla,evaneschneider/cholla,evaneschneider/cholla,evaneschneider/cholla,evaneschneider/cholla