make cached data mutable

dune/curvedgeometry/curvedgeometry.hh

@@ -492,8 +492,8 @@ namespace Dune
     std::vector<GlobalCoordinate> vertices_;
 
     // some data optionally provided
-    Std::optional<bool> affine_;
-    Std::optional<FlatGeometry> flatGeometry_;
+    mutable Std::optional<bool> affine_;
+    mutable Std::optional<FlatGeometry> flatGeometry_;
   };