Free Trade: a trade policy to choose any resource you want

Add "Free Trade" trade policy, which is unlocked by tech "Offworld trade companies". Adopting this policy, you can choose what resource your trade value will exchange by switching the free trade edicts. Trade value is divided equally among all the resources being traded, and energy credits are always traded. --Trade value conversion rate: 1 trade value = 1 energy credits = 1 minerals = 1 food = 0. 5 consumer goods = 0. 25 alloy = 0. 5 unity = 0. 3753 research = 0. 033333 strategic resources = 0. 01667*3 rare resources ( zro, dark matter, living metal ). --For example, if the trade of minerals, food, and consumer goods is opened by edicts, each trade value will be exchanged for 0. 25 energy credits +0. 25 minerals +0. 25 food +0. 125 consumer goods. Corresponding changes are made to civics, federations and traditions.