migrate the C++ _<name> modules directly into the dune namespace

This allows them to be loaded without loading the corresponding Python
mdoule, avoiding circular dependencies.

CMakeLists.txt

 add_python_targets(istl
   __init__
 )
-
-dune_add_pybind11_module(NAME _istl CMAKE_GUARD dune-istl_FOUND)

__init__.py

 from .. import common
 
-from ._istl import *
+from .._istl import *

_istl.cc

-// -*- tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
-// vi: set et ts=4 sw=2 sts=2:
-#include <config.h>
-
-#include <dune/common/fmatrix.hh>
-
-#include <dune/istl/bcrsmatrix.hh>
-#include <dune/istl/bvector.hh>
-
-#include <dune/python/common/fvector.hh>
-#include <dune/python/istl/bcrsmatrix.hh>
-#include <dune/python/istl/bvector.hh>
-#include <dune/python/istl/operators.hh>
-#include <dune/python/istl/preconditioners.hh>
-#include <dune/python/istl/solvers.hh>
-
-#include <dune/python/pybind11/pybind11.h>
-
-PYBIND11_MODULE( _istl, module )
-{
-  // export solver category
-  pybind11::enum_< Dune::SolverCategory::Category > solverCategory( module, "SolverCategory" );
-  solverCategory.value( "sequential", Dune::SolverCategory::sequential );
-  solverCategory.value( "nonoverlapping", Dune::SolverCategory::nonoverlapping );
-  solverCategory.value( "overlapping", Dune::SolverCategory::overlapping );
-
-  // export block vector with block size 1
-  typedef Dune::BlockVector< Dune::FieldVector< double, 1 > > Vector;
-  Dune::Python::registerBlockVector< Vector >( module );
-
-  // export linear operator, preconditioners, and solvers for blockvectors with block size 1
-  pybind11::class_< Dune::LinearOperator< Vector, Vector > > clsLinearOperator( module, "LinearOperator" );
-  Dune::Python::registerLinearOperator( clsLinearOperator );
-  Dune::Python::registerPreconditioners( module, clsLinearOperator );
-  Dune::Python::registerSolvers( module, clsLinearOperator );
-
-  // export BCRS matrix with block size 1x1
-  typedef Dune::BCRSMatrix< Dune::FieldMatrix< double, 1, 1 > > Matrix;
-  Dune::Python::registerBCRSMatrix< Matrix >( module );
-
-  // export matrix-based preconditioners for BCRS matrix with block size 1x1
-  Dune::Python::registerMatrixPreconditioners< Matrix >( module, clsLinearOperator );
-}