change the default grid used for the lens problem back to Dune::YaspGrid

this is because the reference solution changes for newer versions of
dune-alugrid and one of the main purposes of the lens problem is to
allow comparison with Dumux relatively easily. (Dumux usese YaspGrid
for its version of the lens problem.)

examples/problems/lensproblem.hh

@@ -65,6 +65,9 @@ NEW_PROP_TAG(LensUpperRightZ);
 // Set the problem property
 SET_TYPE_PROP(LensBaseProblem, Problem, Ewoms::LensProblem<TypeTag>);
 
+// Use Dune-grid's YaspGrid
+SET_TYPE_PROP(LensBaseProblem, Grid, Dune::YaspGrid<2>);
+
 // Set the wetting phase
 SET_PROP(LensBaseProblem, WettingPhase)
 {