[patches] Update istl patch to 2.6

patches/README

-- If the AMGBackend should be used without SuperLU as coarse grid solver, it can
-  be benefitial to decrease the corresponding tolerance. To do so, apply the
-  patch istl-2.4.1.patch in your directory containing
-  dune-istl 2.4.1, e.g.:
-  patch -p1 <../dumux/patches/istl-2.4.1.patch
-

patches/README.md

+If the AMGBackend should be used without SuperLU as coarse grid solver, it can
+be benefitial to decrease the corresponding tolerance. To do so, apply the patch
+
+ - `istl-2.6.patch` in your directory containing dune-istl 2.6
+   ```bash
+   patch -p1 <../dumux/patches/istl-2.6.patch
+   ```

patches/istl-2.4.1.patch → patches/istl-2.6.patch

 diff --git a/dune/istl/paamg/amg.hh b/dune/istl/paamg/amg.hh
-index 56b1d8d..d672756 100644
+index b4b90a97..5bd66cda 100644
 --- a/dune/istl/paamg/amg.hh
 +++ b/dune/istl/paamg/amg.hh
-@@ -526,13 +526,13 @@ namespace Dune
-               // We are still participating on this level
+@@ -523,14 +523,14 @@ namespace Dune
+               // in order to ensure that we fulfill the alignement requirements
                solver_.reset(new BiCGSTABSolver<X>(const_cast<M&>(matrices_->matrices().coarsest().getRedistributed()),
                                                    *scalarProduct_,
 -                                                  *coarseSmoother_, 1E-2, 1000, 0));
@@ -11,10 +11,11 @@ index 56b1d8d..d672756 100644
              else
                solver_.reset();
            }else
-             solver_.reset(new BiCGSTABSolver<X>(const_cast<M&>(*matrices_->matrices().coarsest()),
-                                                 *scalarProduct_,
--                                                *coarseSmoother_, 1E-2, 1000, 0));
-+                                                *coarseSmoother_, 1E-12, 1000, 0));
-         }
-       }
- 
+           {
+               solver_.reset(new BiCGSTABSolver<X>(const_cast<M&>(*matrices_->matrices().coarsest()),
+                   *scalarProduct_,
+-                  *coarseSmoother_, 1E-2, 1000, 0));
++                  *coarseSmoother_, 1E-12, 1000, 0));
+             // // we have to allocate these types using the rebound allocator
+             // // in order to ensure that we fulfill the alignement requirements
+             // using Alloc = typename A::template rebind<BiCGSTABSolver<X>>::other;