Extend LineZone to filter out miscounts

[grzegorz-roboflow]

Description

LineZone is checking if line was crossed by comparing detection position relative to the line across 2 frames. This is resulting in miscounts when bbox flickers.

Below video shows difference in counts between current develop and this PR.

traffic_side_by_side.mp4

In this PR new init param is added called max_linger, which is by default set to 1 to ensure unchanged default behavior. When max_linger is set to a number greater than 1 then detection must cross on all frames in order to be counted. This results in slight delay (i.e. when max_linger is set to 10 on 10fps video then objects will be counted with 1s delay) however more accurate counts.

Type of change

• New feature (non-breaking change which adds functionality)

How has this change been tested, please provide a testcase or example of how you tested the change?

Added unit tests

Any specific deployment considerations

N/A

Docs

I updated LineZone init docstring.

[LinasKo]

Hi @grzegorz-roboflow,

I'd witnessed this but hadn't verified it. Very nice contribution yet again! I'll take some time to cehck it, especially regarding how the model IDs / edge cases.