Allow optional prediction mask for pixel classification

[imagejan]

Ilastik offers the possibility (both in the UI and from the CLI) to provide a mask in addition to the raw data to reduce the data blocks to be processed when providing sparse data.

Citing @stuarteberg from the ilastik mailing list:

If you have, say, three raw data images to process, and three
corresponding masks, provide the lists of filenames preceded by --raw_data
and --prediction_mask, like this:

./run_ilastik.sh --headless \
  --project=MyProject.ilp \
  --raw_data my_raw_data_1.png my_raw_data_2.png my_raw_data_3.png \
  --prediction_mask mask_1.png mask_2.png mask_3.png

I suggest adding an optional input to the Ilastik Headless (Pixel classification) node, allowing to choose an image column that contains a prediction mask.