Merge branch 'feature/add-istl-patch-2.6' into 'master'

[patches] Update istl patch to 2.6

See merge request !1454 (merged)

(cherry picked from commit 4c6adfa4)

d7803af7 [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
-