load_mot() no longer adapts to MOT17 det.txt files

[capstayn]

Hi,
First of all, great work. Thanks for making it available to the public.

It seems that MOT17 datasets have changed since the code for load_mot() in util.py was written. Now the sequences from detectors FRCNN and SDP only provide det.txt files with 7 columns (no information for classes) and load_mot() fails here.

I fixed it by calling load_mot() in mot17.py with flag with_classes=False. Just wanted to let you know to check that the fix is ok and to suggest to update these demos accordingly, if you reckon appropiate.