Basic Paddock in cities can't be upgraded

[artemgur]

The Basic Paddock building is marked as a final tier, Next tiers exist in the mod files, but are unbuildable.

To see it in game, start the game at 1066 bookmark and click on any city.

[Saeveo]

@artemgur It's being caused by the special camel stables for desert cities, I think. In common/buildings/00_CK2Plus_city.txt, the following buildings:

ct_desert_stable_2
ct_desert_stable_3
ct_desert_stable_4
ct_desert_stable_5

need to have:

potential = {
    terrain = desert
}