Deprecate the file function.hh

It contains the two base classes `Function` and `VirtualFunction`.
In downstream codes, these should be replaced by C++ function objects,
`std::function` etc.

CHANGELOG.md

@@ -3,6 +3,10 @@
 - Previous versions of dune-common imported `std::shared_ptr` and `std::make_shared`
   into the `Dune` namespace.  dune-common-2.8 stops doing that.
 
+- The file `function.hh` is deprecated.  It contained the two base classes
+  `Function` and `VirtualFunction`.  In downstream codes, these should be
+  replaced by C++ function objects, `std::function` etc.
+
 ## build-system
 
 - Remove the CMake function deprecated `inkscape_generate_png_from_svg`.

dune/common/function.hh

@@ -3,6 +3,8 @@
 #ifndef DUNE_FUNCTION_HH
 #define DUNE_FUNCTION_HH
 
+#warning This file is deprecated!  Use C++ function objects and std::function stuff instead!
+
 #include <utility>
 
 #include "typetraits.hh"