[API BREAK] gf : rename namespace gf->gfs

- 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.
[API BREAK] gf : mesh_pt into []

- All access to grid is now using the [] operator.
() always returns const, and is an evaluation over the domain.

- Now :
 * () is always a call to evaluator (or a lazy expression).
 * mesh_pt accepted by [] , both const and non const,
and not by ().

- Ported the libs (fourier et al.) but this break API.
[API BREAK] rename directory gf -> gfs
gf: fix upper bound in out of bounds check in linear mesh