Add Bow histogram computation

[ovysotska]

This PR adds the Python code to:

• train the bag of words vocabulary
• extract image histograms based on pre-trained bag of words vocabulary
• some tests :smile:

In the testing mode, the code takes a folder with images and returns the np.array of Bow features stored in the specified file.

The computed features can be used in the current codebase further, by first turning them into proto files, and then the full pipeline for image_sequence_localizer can be used as usual.

[ovysotska]

Hey @niosus would you have time to review this one?