Fix expression template for gf & std::vector

- add move semantics in expression template.

for gfs :
- rm descriptor
- Remove data from expr, rewrote assignment
  which now iterates on the mesh, simply.
  RHS models now FunctionOnMesh
- This fix the multiplication bug of gf
- Remove assign_no_resize/assign_with_resize from proxy
[API change] gf : factories -> constructors

- Make more general constructors for the gf.
gf( mesh, target_shape_t)
- remove the old make_gf for the basic gf.
- 2 var non generic gf to be removed.
gf: Work on documentation
gfs: clean evaluator,, add tensor_valued

- add a simple vertex test.
- clean specialisation
working