ptycho: swap stack to/from GPU to enable larger datasets in a single GPU
@karpov This can enable larger datasets (e.g. 6K near-field) when it does not make sense to distribute the projections (in far field, the projections can be separated by sample area, but this is usually not the case for near-field as there is little displacement).
This should work in a similar way to holotomo (need at least 3 active stacks, one actively processed, one incoming and one outgoing from the GPU)