Optimize "insert missing frame"

allows to insert missing frames inside trajectory when exporting it to metric txt file. Missing frames are searched for automatically.

1. Check the robustness (e.g. minimum number of persons considered) also for jittering trajectories (only colored caps) or/and
2. try a possibility to use the optical flow between successive frames for measuring the pixel distance between two images to see a doubling or tripling or ... of the most moving pixel areas (people)