segment: add processing with external command

[imagejan]

An optional "pre-processing" step could be processing the list of files with an external command, e.g. a Fiji script called from the command line. This allows running an arbitrary segmentation while still using the filter and sample functionality of this repository.

The best entry point is probably the source folder (as it's a single parameter), but also the list of files would be possible.

Use case:

• Run Fiji script to call Weka Trainable Segmentation, write output masks to new folder
• Return the list of files, so we can hand it over to _process_tif, with an option to skip the segment step (i.e. only run label).
• Process filter and sample as usual.