WIP: Helper method to compute the matrix blocklevel

The following is only a prototype to probe whether the general idea is well-received by the dune-istl community. See the end of the text for a few things that need to be improved.

All ISTL vectors and matrices have to export the 'blocklevel' integer. It is used to determine the nesting depth of a given vector or matrix.

As it turns out, the vector blocklevel field is never actually used within dune-istl. The matrix blocklevel is used a few times. However, with C++11 and beyond it is quite easy to compute the vector and matrix nestings depths without requiring the blocklevel field from each and every vector and matrix implementation.

This patch adds a little helper method matrixBlockLevel that computes the nesting depth of a given matrix type. The method is contained in the Imp namespace, to keep it out of the official dune-istl API. That is a decision to be debated, however.

Before merging, the following details should be improved:

• The new method uses if constexpr, which is not allowed in the Dune core yet.
• It uses MatrixType::block_type. A more general implementation would use the return type of operator[].operator[] here.

dune/istl/eigenvalue/test/matrixinfo.hh

@@ -11,6 +11,7 @@
 #include <dune/common/exceptions.hh>  // provides DUNE_THROW(...), Dune::Exception
 #include <dune/common/fvector.hh>     // provides Dune::FieldVector
 
+#include <dune/istl/blocklevel.hh>
 #include <dune/istl/bvector.hh>          // provides Dune::BlockVector
 #include <dune/istl/superlu.hh>          // provides Dune::SuperLU
 #include <dune/istl/preconditioners.hh>  // provides Dune::SeqGS
@@ -69,7 +70,7 @@ public:
   {
     // assert that BCRSMatrix type has blocklevel 2
     static_assert
-      (BCRSMatrix::blocklevel == 2,
+      (Dune::Imp::matrixBlockLevel<BCRSMatrix>() == 2,
        "Only BCRSMatrices with blocklevel 2 are supported.");
 
     // assert that BCRSMatrix type has square blocks