[dynamic polymorphism] specialize category(...) for getSolverCategory()

getSolverCategory() is used by the AMG communication layer. For convinience
we add this new specialization of category(...)

dune/istl/solvercategory.hh

@@ -33,6 +33,12 @@ namespace Dune {
     }
 
 #ifndef DOXYGEN
+    template<typename OP>
+    static Category category(const OP& op, decltype(op.getSolverCategory())* = nullptr)
+    {
+      return op.getSolverCategory();
+    }
+
     template<typename OP>
     static Category category(const OP& op, decltype(op.category)* = nullptr)
     {