Fix cost

[ydup]

Hi, if the abs(action)=100 > self.state[index+STOCK_DIM+1]=110, the cost at this step will be 0 because the state[index+STOCK_DIM+1] is updated before cost calculation.

self.state[index+STOCK_DIM+1] -= min(abs(action), self.state[index+STOCK_DIM+1])
self.cost +=self.state[index+1]*min(abs(action),self.state[index+STOCK_DIM+1]) * \
                 TRANSACTION_FEE_PERCENT

So I move the cost before updating the state