[Enhancement] Code (modules) reorganisation + Main executable

[lgervasoni]

In order to better understand the framework, the code needs to be reorganized. Modules need to be correctly separated.
As well, a main executable should be added, something like the following:
python urbansprawl [method] [arguments]
where the method can be either the OSM processed data, the spatial sprawl indices, or the population downscaling.

[lgervasoni]

The issue will be split