[1.27c] Supply base should not be allowed as a start tile for new zone

[alocritani]

You cannot build a supply base in a city. --> correct!



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You cannot build a supply base and at the same time create a zone --> correct!



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But... you CAN create a zone on a tile with a supply base! --> this should not be allowed if a tile contains a rural asset

By the way, if you create a zone centered on the supply base, you cannot update that supply base anymore!