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