Add "undefined" as an option for generator fuel

Author: ovasios

src/ps/psoutput.cpp

@@ -221,9 +221,7 @@ PetscErrorCode PSSaveSolution_MATPOWER(PS ps, const char outfile[]) {
           break;
         case GENFUEL_UNDEFINED:
         default:
-          SETERRQ(PETSC_COMM_SELF, PETSC_ERR_FILE_OPEN,
-                  "Generator fuel type (%d) not understod", gfuel);
-          CHKERRQ(ierr);
+          fprintf(fd, "\t\"undefined\"");
           break;
         }
         fprintf(fd, ";\n");