Fix a __repr__ issue with the new gf (issue #112)

  modified:   pytriqs/base/gf/local/gf_imfreq.py
  modified:   pytriqs/base/gf/local/gf_imtime.py
  modified:   pytriqs/base/gf/local/gf_legendre.py
  modified:   pytriqs/base/gf/local/gf_refreq.py