Include every output paths into the prepare_output_folder() function

[delhomer]

This PR simplifies the postprocess API, in the sense that we manage the output folders in a similar fashion than the preprocessed folders.

We consider now new output folders:

• predicted_labels for model predictions;
• predicted_geometries (geographical datasets) for predictions as vectors;
• predicted_rasters (geographical datasets) for predictions as rasters.

The model and instance of reference are included in this refactoring.

Concretely, the prepare_output_folder() function (deeposlandia/utils.py) has been updated and returns now a dictionary.