* Registry for RoI Box Predictors

- Add a registry ROI_BOX_PREDICTOR
- Use the registry in roi_box_predictors.py, replacing the local factory
- Minor changes in structures/bounding_box.py: when copying a box with
fields, check if the field exists
- Minor changes in logger.py: make filename a optional argument with
default value of "log.txt"

* Add Argument skip_missing=False