Use cudipy for GPU-based acceleration

[arokem]

We are already enabling GPU-accelerated tractography via the GPUStreamlines project.

I wonder if we could push this even further by using cudipy, e.g., for registration. It looks like maybe cudipy directly translates the API of syn registration, for example: https://github.com/dipy/cudipy/blob/master/cudipy/align/imwarp.py, in a way that we could ducktype the inputs and outputs as needed.

Might also be a thing that needs to be done upstream in dipy itself, though.