Because tests will be run after model_server is installed as a package, they revert to importing model_server.base.*

Adapted base segmentation worklow and API endpoint for generic semantic segmentation model; made dummy model this type too

(1) slicing to a single-channel image now returns its own accessor object; (2) channel slicing now happened at image accessor level, i.e. model is agnostic; (3) TIFF export is now a utility method, not a subclass of accessor