Support multiple detectors

[cwmeijer]

Our work flow should support using multiple detectors. We should be able to run YOLO, SSD and the face detector all independently from on another. After that we can run all croppers and classifiers on it.
Because in the current state we can only use a single detector (not counting the face detector which we currently run in a later step), the workflow is too dependent on the performance of this single detector.