Merge branch 'feature/fix-dense-matrix' into 'master'

[fix] Remove coumns_, probably a copy-n-paste artifact



See merge request !33

dune/istl/matrix.hh

@@ -406,7 +406,6 @@ namespace MatrixImp
       : window_(nullptr,0)
       {
         data_ = nullptr;
-        columns_ = 0;
         i = 0;
       }