Add pade approximants

The old files have been reorganized. The pade_appximants class is put
in the triqs/utility directory. The gf-related pade() function is in
triqs/gf/local.

  new file:   triqs/utility/pade_approximants.hpp
  new file:   triqs/gf/local/pade.cpp
  new file:   triqs/gf/local/pade.hpp
Add set_from_pade in python gf refreq

Checked in a notebook. Seems ok.

  modified:   pytriqs/base/gf/local/functions.pxd
  modified:   pytriqs/base/gf/local/refreq.pyx