deprecated gfs files removed
correction of a bug in imtime (evaluator_fnt_on_mesh) + corresponding test added
doc: small tutorial for the GF's
Fixed antiperiodicity property of imtime

- the condition p%2 ==1 was wrong if p<0 (never true!)
- added corresponding test (gf_notail)

Conflicts:
	triqs/gfs/imtime.hpp

Fixed by O.P. : already fixed in Laura's pull request ...
Added density for scalar_valued gf (and test thereof)