Download and Unzip

Hi,

The number of packages/classes that relies on pretrained models is increasing. Follow below a rough estimation:

• bob.ip.tensorflow_extractor (3 classes)
• bob.ip.caffe_extractor (2 classes)
• bob.ip.pytorch_extractor (1 class as far as I know)
• bob.ip.dlib (1 class)

All of these classes implement their own download method, which consists in to:

• Download from one or more sources
• Unzip/Untar to some directory.

This is a good feature candidate for bob.extension. Would avoid some code repetition.

What do you think?