What's Changed
* Added background masking functionality as described in README [here](https://github.com/levan92/deep_sort_realtime#background-masking)
* Code fixes in response to [deprecation](https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations) of `np.float` in `numpy`
* Exposed `max_iou_distance` as an argument in `deep_sort_realtime.deepsort_tracker.DeepSort` object
* Exposed `gating_only_position` as an argument in `deep_sort_realtime.deep_sort.tracker.Tracker` (if flagged `True`, during gating, only x y position is taken into account, instead of the usual x, y, aspect ratio and height)
* Minor fixes and README updates