add default parameters for InnerStride/OuterStride to be

able to simply write OuterStride<> instead of OuterStride<Dynamic>

doc/snippets/Map_outer_stride.cpp

@@ -1,5 +1,3 @@
 int array[12];
 for(int i = 0; i < 12; ++i) array[i] = i;
-cout << Map<MatrixXi, 0, OuterStride<Dynamic> >
-         (array, 3, 3, OuterStride<Dynamic>(4))
-     << endl;
+cout << Map<MatrixXi, 0, OuterStride<> >(array, 3, 3, OuterStride<>(4)) << endl;