Preprocessing: Add WSI prefetching

[FWao]

When the files containing compressed WSIs are located in network drives, the random accesses to extract tiles might be slow. The preload_wsi option first makes a copy of the WSI file in a temporary local directory and can massively speed up the tile extraction.

Please update your config.yaml file:

preprocessing:
  ...
  preload_wsi: true

[FWao]

@akutzer Thank you for pointing that out. I propose this version then. Still untested however.