Add Legendre transforms and tail calculations

  new file:   pytriqs/base/gf/local/functions.pxd
  new file:   triqs/gf/local/legendre_matsubara.cpp
  new file:   triqs/gf/local/legendre_matsubara.hpp
  renamed:    triqs/gf/local/density.cpp -> triqs/gf/local/functions.cpp
  renamed:    triqs/gf/local/density.hpp -> triqs/gf/local/functions.hpp
  modified:   pytriqs/base/gf/local/gf.pyx
  modified:   pytriqs/base/gf/local/gf_legendre.py
  modified:   pytriqs/base/gf/local/imfreq.pyx
  modified:   pytriqs/base/gf/local/imtime.pyx
  modified:   pytriqs/base/gf/local/legendre.pxd
  modified:   pytriqs/base/gf/local/legendre.pyx
  modified:   triqs/gf/legendre.hpp
  modified:   triqs/gf/local/fourier_matsubara.cpp
  modified:   triqs/gf/local/fourier_matsubara.hpp
  modified:   triqs/gf/test/gfv2.cpp
  modified:   triqs/utility/legendre.hpp
  deleted:    triqs/gf/local/no-built/legendre_matsubara.cpp
  deleted:    triqs/gf/local/no-built/legendre_matsubara.hpp
  deleted:    pytriqs/base/gf/local/density.pxd
  deleted:    pytriqs/base/gf/local/fourier.pxd