Refactorize parcelgen to be more like a python package

[macisamuele]

The goal of this PR is to make possible to use parcelgen as CLI tool.

To achieve this we need to:

• extract the main logic into a python function rather than in the if __name__ == '__main__' statement
• add setup.py file

The additional changes on the tool are meant mainly to make it a bit more pythonic (if this creates more confusion I can undo those)

Those changes are needed to achieve COREMOBAPI-217 (internal ticket)