Add offers to civic structures

[Jeremy-Gaillard]

The goodrelations cookbook has examples of complex pricings that require both Offers and PriceSpecifications to be modelled (quantity discounts most notably). These are the kind of pricing that I am currently trying to model for a parking facility.

In mobivoc, the Civic structure class only has Price specification in its range, which prevents the implementation of these complex pricings. Therefore, I suggest adding offers to the Civic structure class to fix this issue.