Add upper solver w/o additional copy

dune/matrix-vector/triangularsolve.hh

@@ -36,7 +36,22 @@ namespace MatrixVector {
     x = 0;
     Vector r = b;
     if (transpose) {
-      // TODO: not yet handled
+      for (auto it = U.beforeEnd(); it != U.beforeBegin(); --it) {
+        size_t i = it.index();
+        auto cIt = it->beforeEnd();
+        assert(cIt.index() == i);
+        auto diagonal = *cIt;
+        if (ignore != nullptr and (*ignore)[i].all())
+          continue;
+        x[i] = r[i] / diagonal;
+
+        cIt--;
+        for (; cIt != it->beforeBegin(); --cIt) {
+          size_t j = cIt.index();
+          assert(j < i);
+          r[j] -= *cIt * x[i];
+        }
+      }
     } else {
       for (auto it = U.beforeEnd(); it != U.beforeBegin(); --it) {
         size_t i = it.index();