create script for running frame creation on different datasets

[uoefeb]

• slightly changes how the sampling scripts set the save directory (trailing "/"s on a path could cause problems before)
• creates script for running frame creation on the different data sets (see setup_data.sh)
• update gitignore to block out other videos and frames

Assumes we'll lay out the rest of our videos in the following structure

/data/{train_val, test}/
  videos/
  *_frames/
  *.json

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btw, I've got transnet running overnight.