Localisation.adresse field should not be unique

[jmaupetit]

Purpose

At the time of writing, it has been reported that we cannot create two distinct stations using the same address and different coordinates. As addresses could be sparse data, we should be more flexible on this and allow two stations to be referenced with the same address (e.g. street, zip code and city but no street number).

Proposal

1. remove the uniqueness constraint on the Localisation.adresse field,
2. add (pick one):
   • a unique-together constraint on the Localisation.(adresse, coordonneesXY) fields couple, or,
   • a unique constraint on the Localisation.coordonneesXY field