Now the tail have a fixed size. It actually makes everything simpler. I took
order_min = -1 and order_max = 8. This makes the tails compatible with the
previous implementation. However we might want to change this to something like
-10, 10 so that they are self-contained. This commit should also fix issue #11.
- 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.
