Pass notebook filename and not path to load an ilastik model; assumes project files are in conf.server.paths['ilastik']

(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

 Conflicts:
	tests/test_api.py