Update the 50 m threshold in the performance requirements

[mrksbrg]

The 50 m threshold for pedestrian detection and recognition in the performance requirements does not match the ODD. If ego car moves at 70 km/h, a TTC of 4 s means that the pedestrian can be almost 80 m away.

[mrksbrg]

This is related to Issue #8. Since we replaced accuracy with true positive rate in SYS-PER-REQ1, SYS-PER-REQ2 is now redundant. We solve this by specifying requirements on TPR and FNR at different distances. We replace 50 m with 80 m in all cases except in SYS-PER-REQ2 where we keep 50 m. We motivate this by the presence of a driver - the driver is reponsible for braking ego car if a pedestrian shows up, thus we measure FNR closer to the vechicle.

[mrksbrg]

At the same time, align ego car and ego vechicle throughout the SRS.