Improve create_dataset, video_to_slomo

[tobbez]

• In both: Use subprocess.check_call instead of os.system. This means even file names with special characters (e.g. ") will work correctly.
• In create_dataset: Increase the number of digits used in frame file names. The previous number would cause frames to appear out of order when using them in training if a video used for the data set had more than 9999 frames.