Fix speed of hdf archives (fix issue #49)

The keys are now cached all over. In principle this makes it fast to check
wether a key is present or not. A lot faster than through the h5py _group.keys().
Tests pass and in principle this addresses the github issue #49.

  modified:   pytriqs/base/archive/hdf_archive_basic_layer_h5py.py
Delete useless gf_basic_tools.pxd

  deleted:    pytriqs/base/gf/local/gf_basic_tools.pxd
Add python interface for real freq GF

  new file:   pytriqs/base/gf/local/mesh_refreq.pyx
  new file:   pytriqs/base/gf/local/refreq.pxd
  new file:   pytriqs/base/gf/local/refreq.pyx
  modified:   pytriqs/base/gf/local/gf.pxd
  modified:   pytriqs/base/gf/local/gf.pyx
  modified:   pytriqs/base/gf/local/gf_refreq.py
  modified:   pytriqs/base/gf/local/imtime.pxd
  modified:   pytriqs/base/gf/local/mesh_imtime.pyx
  modified:   triqs/gf/imfreq.hpp
  modified:   triqs/gf/imtime.hpp
  modified:   triqs/gf/local/fourier_matsubara.cpp
  modified:   triqs/gf/local/legendre_matsubara.cpp
  modified:   triqs/gf/meshes/linear.hpp
  modified:   triqs/gf/refreq.hpp
  modified:   triqs/gf/retime.hpp
Fix non-status message in CMakeLists.txt

  modified:   CMakeLists.txt
Fix option name in impl_plot.py

  modified:   pytriqs/base/gf/local/impl_plot.py
Adapt developer_manual to latest library changes

  modified:   doc/developer_manual/CMakeLists.txt
  modified:   doc/developer_manual/arrays/CMakeLists.txt
  modified:   doc/developer_manual/clef/assign.rst