Structure of the package

[felix-schott]

Description

Two things that I'm unclear about:

1. Not quite sure what the purpose of common.py is - can this be deleted?
2. I would also recommend to move utility functions to a utils.py file and further put all CLI handlers in the cli.py file, with a clear separation of CLI handler (command line interface) and library function (python interface). E.g. not quite clear sure why download_buildings also contains CLI handlers.