Configs + Code cleanup

[charlesyz]

Hello! Great work on the project.

I wanted to clean up the code a bit so I and others can build upon this repo for future tracking efforts

• Use YAML config files
• Use Dependency injection to select which algorithms and covariances to use (see algorithms.py and covariance.py)
• Isolate IOU/distance metric calculation and matching algorithms into separate functions in algorithms.py
• Move util functions to utils/geometry_utils.py and generic_utils.py

I tested the code with configurations in the readme (config/AB3DMOT.yaml and config/mahalanobis.yaml) and was able to reproduce the results cited in the README.

[eddyhkchiu]

Thank you very much for the update suggestions! I will keep this pull request open since we have more than one pull request which may conflict with each other.