Linkstateior2r refac

[jvendries]

Changes to in the linkstateior2rRefac branch:

• Replace model$U and model$U_d with the stateior object, TwoRegion_Summary_DomesticUsewithTrade (e.g.)
• Modify model build and validation functions to work with the TwoRegion_Summary_DomesticUsewithTrade values.
• See this Jira comment for additional details on the reasoning behind the changes.

[bl-young]

@jvendries confirming that running printValidationResults() shows no errors correct? (notwithstanding that domestic testing for two-region models is the same as the full model testing, since U_d has been replaced)

[jvendries]

@jvendries confirming that running printValidationResults() shows no errors correct? (notwithstanding that domestic testing for two-region models is the same as the full model testing, since U_d has been replaced)

@bl-young Yes, printValidationResults() shows no errors