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Closes #93

This made sense some time ago. Nowdays, the whitewhale runner seems to
not really be reliable, so tagging jobs with it doesn't gain much.

Relates #45

Relates #45

Relates #45
Relates #93

This should closes  #90

- This commit introduces optional dependencies (namely ffmpeg):
  mat2 will spit a warning when trying to process an .avi file
  if ffmpeg isn't installed.
- Since metadata are obtained via exiftool, this commit
  also refactors a bit our exfitool wrapper.

Fixes issue #61

This should fix the intermittent failures, thanks
to @pollo for the tip

refactor

- the `src` folder is now `libmat2`
- the `main.py` script is now `mat2.py`