gcc has a pb because the template mesh<Variable,Opt>
has the name same as the gf mesh method (!).
Clang is fine however on this...
Solution : rename the template mesh<...> to gf_mesh...
Not very elegant, but ok.
- necessary otherwise the class gf and the namespace have
the same name, leading to some confusion, and need to qualify
some functions (specially on gcc).
Same naming conventions as arrays.