added GRAVITY_COUPLE flag to chose where to couple gravity to hydro
added GRAVITY_COUPLE flag to chose where to couple gravity to hydro

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