Postprocessing improvement

Added

• --nb-tiles-per-image as a new argument for datagen command.
• A progress bar for inference processes (#153)

Changed

• utils.prepare_output_folder() returns now a dictionary of all useful output paths
• Some dependency updates (Tensorflow, opencv, pillow, keras, daiquiri)
• The preprocessing has been modified for geographic datasets: -t, -v and -T now
  refer to raw images, the amount of preprocessed tiles being obtained by a combination
  of --nb-tiles-per-image and these last arguments.
• The tile grid becomes optional for postprocessing (#155).

Fixed

• Draws without replacement instead of with replacement in the case of preprocessing
  of geographic dataset testing images (np.random.choice wrong parameterization). #146

Security

• pillow was updated to 7.1.1 (moderate severity vulnerability alert for
  pillow<6.2.2)

Removed

• sys.exit statements (#150)