Add iou_complement as a configurable alternative distance function to center_distance (for detection and tracking evaluation)

[sbarkby]

This PR introduces the Intersection Over Union complement (1.0 - IOU) as an alternative distance function for determining matches and the calculation of metrics, such as MOTP. The use of this distance function has been made configurable in the detection_cvpr_2019.json and tracking_nips_2019.json config files for detection and tracking evaluation respectively. Fortunately the codebase was already setup to cater for alternative distance functions, so the coding change proposed here is minimal.

Also note that this PR poses no change to the default operation of the evaluator, it will continue to use "center_distance" as it's distance function by default.

[whyekit-motional]

Thanks for this feature @sbarkby!

I've done an initial pass on PR (and will look deeper into your code in subsequent passes)

[sbarkby]

Hi @whyekit-motional , thanks for the review, have made the changes you suggested, let me know if you need anything else.

Cheers. :)

[sbarkby]

Hi @whyekit-motional, just checking in, any updates?

[sbarkby]

Hi @whyekit-motional, just checking in, any updates? Does this need to be transferred to a different reviewer?