added temperature floor when doing the ChemistrGPU step
added temperature floor when doing the ChemistrGPU step

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