2R models: Consider adding export residual and international trade adjustment to Use

[WesIngwersen]

2R models from stateior require two additional final demand components for balancing, an export residual (defined in #298) and also an ITA - international trade adjustment which is a Use table - Import Matrix difference where the value of domestic inputs like insurance, wholesale, etc is removed from import values.
These are only included in the DomesticUsewithTrade data from stateior.

The model Use tables (both U and U_d) and further procedures based on them would be simplified if they are included but the implications need to be assessed.

[jvendries]

@WesIngwersen To clarify, are you suggesting that we add the ITA and export residuals:

1. To the final model$U object, which is not used either by 1R or 2R models to generate the direct requirements table and its derivatives, such that these additions would essentially be for model export purposes;
2. To the intermediate model$FinalDemand object, so that these columns used in the commodity output calculations; or
3. Somewhere else? (I don't think you mean the intermediate model$UseTransactions object as that would change the matrix dimensions and interference with all the downstream calculations involving the Make table and derivatives)

Option 1 is fairly straightforward and shouldn't impact and model results. Option 2 is more complicated and may require adjustments elsewhere in the code to make sure model results are not impacted.