Add 1D GPU support

[lgarrison]

Currently, only 2D and 3D transforms were supported on the GPU. During the original implementation, the thinking was that there wouldn't be much advantage to doing 1D on the GPU (IIRC).

However, in nifty-ls, we found that 1D was substantially faster on the GPU once you get to sufficiently large problems (e.g. N_data ~ 10^3, N_freq ~ 10^4). And besides, there may be cases where users have a multi-step workflow where they want to stay on the GPU the whole time.

I imagine this isn't that hard, it just needs to be implemented and tested.