Fix warning

src/mesh/CellType.hpp

@@ -28,6 +28,7 @@ std::string_view name(const CellType& cell_type)
     case CellType::Pyramid: return "pyramid";
     case CellType::Prism: return "prism";
     case CellType::Hexahedron: return "hexahedron";
+    default: return "unknown cell type";
   }
 }