Fix bug OUTPUT_FOLDER is parsed as a list.

[tienthanh1993]

Hello Jonathon,

I've found minor bug in OUTPUT_FOLDER argument which is caused "TypeError: expected str, bytes or os.PathLike object, not list" error when using OUTPUT_FOLDER argument.

expected str, bytes or os.PathLike object, not list
Traceback (most recent call last):
File "***\trackeval\eval.py", line 126, in evaluate
output_fol = dataset.get_output_fol(tracker)
File "***\trackeval\datasets_base_dataset.py", line 49, in get_output_fol
return os.path.join(self.output_fol, tracker, self.output_sub_fol)
File "D:\DevTools\Anaconda3\envs\smartcam-gpu\lib\ntpath.py", line 76, in join
path = os.fspath(path)
TypeError: expected str, bytes or os.PathLike object, not list
Traceback (most recent call last):
File "evaluate/scripts/run_mot_challenge.py", line 91, in
evaluator.evaluate(dataset_list, metrics_list)
File "***\trackeval_timing.py", line 16, in wrap
result = f(*args, **kw)
File "***\trackeval\eval.py", line 177, in evaluate
raise err
File "***\trackeval\eval.py", line 126, in evaluate
output_fol = dataset.get_output_fol(tracker)
File "***\trackeval\datasets_base_dataset.py", line 49, in get_output_fol
return os.path.join(self.output_fol, tracker, self.output_sub_fol)
File "D:\DevTools\Anaconda3\envs\smartcam-gpu\lib\ntpath.py", line 76, in join
path = os.fspath(path)
TypeError: expected str, bytes or os.PathLike object, not list

default OUTPUT_FOLDER was None at https://github.com/JonathonLuiten/TrackEval/blob/master/trackeval/datasets/mots_challenge.py#L22
and parsed as list at
https://github.com/JonathonLuiten/TrackEval/blob/master/scripts/run_mot_challenge.py#L55.
It's similar to other datasets

Thanks for your great work!
Thanh.