Kryoflux RAW : create any missing directories in output path

[renaudguerin]

Small quality of life improvement : when opening Kryoflux RAW files for writing, create any missing directories contained in the filename pattern.

I typically want to keep KF RAW files together in a subdirectory without having to create it manually beforehand (e.g. gw read greatgame/00.0.raw).

Note: I initially made the change at the Image class level but then realised that KF overloads the __enter__ method (and other formats might not benefit as much), so it's now in kryoflux.py only.