GPU performance of expv and expv_timestep could maybe be improved

[ChrisRackauckas]

See #101 . I don't think there was very rigorous testing of how some parts are on CPU and some parts are on GPU, though there is good justification. It would be nice to take a fine-toothed comb to this code and really optimize it, though the current code is likely close enough to optimal that it might not matter.