Load pil stack

There were mixed features in thsi branch, for the lack of time.

• Data loader now loads a folder of PIL files
• It returns a Dask array in case virtual_stack
• Fixed aspect ratio problem for k3d
• Quick workaround for the cupy import problem

requested review from @fima

assigned to @fima

approved this merge request

merged

mentioned in commit 2c9d0b67