stitching: allow to provide external darks / flats
It can happen that users will not have 'reduced_darks' and 'reduced_flats' but their own darks / flats. We should be able to define this from the configuration file.
It can happen that users will not have 'reduced_darks' and 'reduced_flats' but their own darks / flats. We should be able to define this from the configuration file.