Support multiple groups

[almet]

It could be cool that one instance of copanier handles multiple groups.

Also, we might need rework the wording here, to distinguish:

• Ordering entity : A collection of persons that can place orders. It could be families or shared houses, for instance;
• A cooperative : A collection of ordering entities.

I'm not sure about the naming here. Any help is appreciated :-)

[almet]

Names :

• "Foyer" (in french): for people who are ordering on behalf of their house (in english "house" or "home" ?);
• "Groupement" (in french), maybe "Collective" in english for a group of houses.