load model from path

Currently, MLAir expects a trained model to be inside the experiment path inside the model directory. When running a fresh experiment run with no training at all, this will fail as there is no option to refer to an existing model (without copying it into to model directory after initiating the MLAir workflow). Therefore add option to use external model.

• introduce model_path parameter that can be set during ExperimentSetup
• add option to copy the given model inside the experiment path (which should then overwrite the external model_path?)