Copy method 'makeFunction' into namespace Dune::Fufem

The method 'makeFunction' was declared in a namespace 'Functions'.
This leads to unfortunate clashes with code from the dune-functions
module, in particular when using

  using namespace Dune;

This patch introduces a copy of that method in the namespace
Dune::Fufem (never mind the copy: the method is very small).
It also deprecates the method in the old namespace.  That way,
code that currently works still does so in the future, but we
now have a migration path away from that unfortunate 'Functions'
namespace in dune-fufem.

BUG: https://git.imp.fu-berlin.de/agnumpde/dune-fufem/issues/11

CHANGELOG.md

 # Master (will become release 2.8)
 
-- ...
-
 ## Deprecations and removals
 
-- ...
+- The method `makeFunction` has been copied into the namespace `Dune::Fufem`.
+  Previously, it was declared in the namespace 'Functions'. This lead
+  to unfortunate clashes with code from the dune-functions module.
+  Release 2.8 introduces a copy of that method in the namespace
+  `Dune::Fufem`. It also deprecates the method in the old namespace.
+
+# Release 2.7
+
+No changelog available until Release 2.7

dune/fufem/functions/basisgridfunction.hh

@@ -295,6 +295,7 @@ namespace Functions
 {
 
 template<class B, class C>
+DUNE_DEPRECATED_MSG("Use method 'makeFunction' from namespace 'Dune::Fufem' instead!")
 BasisGridFunction<B, C> makeFunction(const B& basis, const C& coeff)
 {
     return BasisGridFunction<B,C>(basis, coeff);
@@ -302,7 +303,16 @@ BasisGridFunction<B, C> makeFunction(const B& basis, const C& coeff)
 
 } // end namespace Functions
 
+namespace Dune::Fufem
+{
+
+template<class B, class C>
+BasisGridFunction<B, C> makeFunction(const B& basis, const C& coeff)
+{
+    return BasisGridFunction<B,C>(basis, coeff);
+}
 
+} // end namespace Dune::Fufem
 
 #endif