Implement realistic trading sheme

[jakobkolb]

so far, trade income is calculated ad-hoc from some crude measures of the trade network between settlements.

def get_trade_income(self):
        self.trade_income = 1./30.*( 1 + self.comp_size/self.centrality )**0.9
        self.trade_income[self.trade_income>1] = 1
        self.trade_income[self.trade_income<0] = 0
        self.trade_income[self.degree==0] = 0
        return self.trade_income

This is hardly realistic.
But a better scheme needs to be properly founded in historical facts and economic theory. Julian Maluck might know more about this.