Fix CMakeLists message with doxygen

  modified:   doc/developer_manual/CMakeLists.txt
Add Fourier for real gf's (part II)

The Fourier transforms have been checked a bit more. Added functions
to generate the Fourier transforms (a new gf is build). They have
been wrapped into python methods.

  modified:   pytriqs/gf/local/functions.pxd
  modified:   pytriqs/gf/local/gf_retime.py
  modified:   pytriqs/gf/local/refreq.pyx
  modified:   pytriqs/gf/local/retime.pyx
  modified:   triqs/gf/local/fourier_real.cpp
  modified:   triqs/gf/local/fourier_real.hpp
Fix documentation for GfReTime and GfReFreq

  modified:   doc/user_manual/green/block/GfReFreq.rst
  modified:   doc/user_manual/green/block/GfReTime.rst
  modified:   doc/user_manual/green/block/green_retime.py