doc: git only in html mode
arrays : minor cosmetics
Work on gf

- clean curry.
- start testing.
mv test gf -> gfs
utility: simplify a bit tuple::apply
Fix #5

- a temp. array in stev had the wrong sign.
arrays: fix some test to avoid valgrind error
clef : new version using lvalues and moving rvalues

- change : all objects are by default
  stored now by reference, not by copy any more.
  Unless the trait force_copy_in_expr is true.
- rvalue refs are moved into the tree
- simplifies a lot the writing of lazy method, objects.
- added a macro for methods
- tests ok. Further check needed to control absence of copies...
- improved documentation