Fix directory automatic creation if missing

src/output/GnuplotWriter1D.cpp

@@ -216,6 +216,8 @@ GnuplotWriter1D::_write(const MeshType& mesh,
     throw NormalError("cannot store both node and cell data in the same gnuplot file");
   }
 
+  createDirectoryIfNeeded(_getFilename());
+
   std::ofstream fout;
 
   if (parallel::rank() == 0) {