gf: imfreq : fix evaluator for n<0

- in positive only case, the evaluator for n<0
was wrong : must shift the integer by -1 for the fermions.
gf : imfreq : clean evaluator on int

- evaluator on int was not using the same algorithm
as matsubara_freq, which can be surprising...
Fix in off-diagonal elements of density()

This should fix the bug noticed by Leonid in off-diagonal
elements of the density matrix.

  modified:   triqs/gfs/local/functions.cpp
Merge branch 'master' of github.com:TRIQS/libs