Implement metrics changes

[timatcambrio]

Changes:

1. Allows for dropping detections in solver submissions that are matched to LOW confidence ground truth labels;
2. Allows for increasing the cost for prediction-ground truth pairs whose distance is greater than the distance tolerance; related to Distance-based matching in metric.py is broken #9.

[favyen2]

Is it intended that costly_dist is disabled in drop_low_confidence_preds even with --costly_dist flag (args.costly_dist is not passed to drop_low_confidence_preds)?

[timatcambrio]

Is it intended that costly_dist is disabled in drop_low_confidence_preds even with --costly_dist flag (args.costly_dist is not passed to drop_low_confidence_preds)?

Nice catch! Thank you, @favyen2!