[MILU] Add dropped elements only to diagonal.

We did add the dropped matrix blocks to the diagonal blocks,
but this is not MILU where oone only modifies the diagonal.
With this patch we fix this behaviour and now only modify the
diagonal of the diagonal block.

opm/autodiff/ParallelOverlappingILU0.hpp

@@ -179,7 +179,15 @@ namespace Opm
             if ( a_ik.index() != irow.index() )
                 OPM_THROW(std::logic_error, "Matrix is missing diagonal for row " << irow.index());
 
-            *a_ik -= sum_dropped;
+            int index = 0;
+            for(const auto& row: sum_dropped)
+            {
+                for(const auto& val: row)
+                {
+                    (*a_ik)[index][index]-=val;
+                }
+                ++index;
+            }
 
             if ( diagonal )
             {