pixel classification: headless: --retrain flag forces a retrain and saves the classifier before doing any batch prediction.
PixelClassificaiton: Headless: Warn the user if his project file contains no classifier and therefore needs retraining.
PixelClassification: Fixed --retrain option (need to unfreeze predictions to force a retrain)
tests: pixel classification: headless test: Dummy project saves the classifier before running the test to avoid a warning message on the console.