Speed up MatrixIndestSet::exportIdx()

Before this patch `BCRSMatrix::addindex()` was used on individual
column indices. This is slow because each insertion does a binary
search although the inserted indices are already sorted. Bulk-inserting
whole rows with `setIndices()` improves on this significantly but still
does a non-necessary sort. The latter is avoided by the new
`BCRSMatrix::setIndicesNoSort()` method.

dune/istl/matrixindexset.hh

@@ -193,8 +193,7 @@ namespace Dune {
 
       for (size_type row=0; row<rows_; row++) {
         std::visit([&](const auto& rowIndices) {
-          for(size_type col : rowIndices)
-             matrix.addindex(row, col);
+          matrix.setIndicesNoSort(row, rowIndices.begin(), rowIndices.end());
         }, indices_[row]);
       }