Replace comment on missing features with NotImplemented

dune/solvers/iterationsteps/projectedblockgsstep.cc

@@ -12,10 +12,12 @@ void ProjectedBlockGSStep<MatrixType, VectorType>::iterate()
     const MatrixType& mat = *this->mat_;
 
     for (size_t i=0; i<this->x_->size(); i++) {
-
-        /** \todo Handle more general boundary conditions */
-        if ((*this->ignoreNodes_)[i][0])
+        auto ignoreCount = (*this->ignoreNodes_)[i].count();
+        if (ignoreCount == BlockSize)
             continue;
+        if (ignoreCount > 0)
+          DUNE_THROW(Dune::NotImplemented,
+                     "Individual blocks must be either ignored completely, or not at all");
 
         bool zeroDiagonal = false;
         for (size_t j=0; j<BlockSize; j++) {