Fix: Starting residual computation with wrong iterate.

dune/matrix-vector/triangularsolve.hh

@@ -16,6 +16,7 @@ namespace MatrixVector {
         auto cIt = it->begin();
         assert(cIt.index() == it.index());
         x[i] = b[i] / *cIt;
+        cIt++;
         for (; cIt != it->end(); ++cIt) {
           const size_t j = cIt.index();
           b[j] -= x[i] * *cIt;