[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.
try to treat the explicit construction of tuple ...
fix for gcc 4.6
start to fix narrowing issue
Fix narrowing int issue on clang 3.3
Work on doc
gfs: clean evaluator, regroup code.