[ENH] add --prefer-gpu

[mattcieslak]

v0.21 has gpu-enabled versions of synthstrip, synthseg, drbuddi and eddy included. Previously only eddy had gpu capabilities and these had to be accessed by a special eddy config json file.

I propose adding a --prefer-gpu flag that will switch each of these to their gpu version. There will still have to be an eddy config json file for specifying all the other options, but the "use_cuda" option will be taken out of the json file and specified on the commandline.

[mattcieslak]

synthseg needs a smarter bounding box workflow like the one already being used by synthstrip. It doesn't fit on an 8gb gpu without some cropping

[mattcieslak]

Or maybe it would be better to have a --gpu [require|try|none] where require mode the will raise an error if the gpu version fails, try will try to use the gpu and fall back to the cpu version if the gpu fails and none won't attempt to use a gpu at all?

I'm just starting to run gpu jobs on an HPC and it looks like they randomly fail a lot