Support decompressing to tar; decompress to jpg by default.

[mattiasegu]

This PR implements decompression to tarball and decompression to .jpg by default.

The following functionalities are still to be implemented:

• [x] Add option to decompress to hdf5
• [x] Add option to decompress to zip
• [x] Save with filename suffix
• [x] Add documentation on decompression mode

[suniique]

An example of decompressed tar/hdf5.

$ tar -tvf /Users/taosun/Documents/shift_dataset/discrete/videos/val/front/img_decompressed.tar | head
drwxr-xr-x  0 taosun staff       0 Oct 23 19:55 eba5-a5bd/
-rw-r--r--  0 taosun staff   78153 Oct 23 19:55 eba5-a5bd/00000000_img_front.jpg
-rw-r--r--  0 taosun staff  109909 Oct 23 19:55 eba5-a5bd/00000001_img_front.jpg
-rw-r--r--  0 taosun staff  121730 Oct 23 19:55 eba5-a5bd/00000002_img_front.jpg