Pass grouping object to make_stats_for_one_grouping instead of its attributes

We are aiming to move some of the logic inside
make_stats_for_one_grouping to the Line object(s).

A first step towards that is to have the Grouping object
as argument to the method, and not merely the key.

For now, we basically extract back the attributes.

This however requires significant overhaul of the unit tests.