Revert change to scopflow base case subproblem setup.

Author: nkoukpaizan

src/scopflow/interface/scopflow.cpp

@@ -695,7 +695,8 @@ PetscErrorCode SCOPFLOWSetUp(SCOPFLOW scopflow) {
     ierr = OPFLOWIgnoreLineflowConstraints(
         scopflow->opflow0, scopflow->ignore_lineflow_constraints);
     CHKERRQ(ierr);
-    ierr = OPFLOWSetModel(scopflow->opflow0, scopflow->subproblem_model);
+    /* Base-case problem model should be POWER_BALANCE_POLAR */
+    ierr = OPFLOWSetModel(scopflow->opflow0, OPFLOWMODEL_PBPOL);
     CHKERRQ(ierr);
     /* Base-case problem solver should be IPOPT */
     ierr = OPFLOWSetSolver(scopflow->opflow0, OPFLOWSOLVER_IPOPT);