Partially revert last patch, and improve the implementation some more

- shorter code
- don't zero out columns, there's no reason to do so
- don't copy rows that get overwritten with zeros anyways

[[Imported from SVN: r4894]]

dune/solvers/iterationsteps/blockgsstep.cc

@@ -57,24 +57,18 @@ void BlockGSStep<OperatorType, DiscFuncType, BitVectorType>::iterate()
             // No degree of freedom shall be ignored --> solve linear problem
             mat[i][i].solve(v, r);
         } else {
-            // Copy the matrix and adjust rhs so that dofs given in ignoreNodes[i]
+            // Copy the matrix and adjust rhs and matrix so that dofs given in ignoreNodes[i]
             // are not touched
-            typename OperatorType::block_type matRes = mat[i][i];
+            typename OperatorType::block_type matRes;
             for (int j = 0; j < BlockSize; ++j) {
                 if ((*this->ignoreNodes_)[i][j]) {
                     r[j] = 0;
 
-                    for (int k = 0; k < BlockSize; ++k) {
-			if (k == j)
-			    continue;
+                    for (int k = 0; k < BlockSize; ++k)
+			matRes[j][k] = (k == j);
+                } else 
+                    matRes[j] == mat[i][i][j];
 
-			matRes[j][k] = 0;
-			// Since r[j] is zero anyway, this changes
-			// nothing but simplifies solving
-			matRes[k][j] = 0;
-		    }
-		    matRes[j][j] = 1;
-                }
             }
             matRes.solve(v, r);
         }