include unitcube2d.dgf in problem-ball

examples/problem-ball.hh

@@ -36,7 +36,7 @@ struct BallData
     else if ( problem_ == 3 )
       dgfFileName << path << "/dgf/cube_hc_32768.dgf";
     else
-      dgfFileName << path << "/dgf/unitcube3d.dgf";
+      dgfFileName << path << "/dgf/unitcube" << dimDomain << "d.dgf";
     return dgfFileName.str();
   }