fourier_real: scalar_valued implementation
fourier_matsubara: still some useless & removed
refreq_imtime.hpp: bug corrected
test of re_im_freq_time.cpp ok
gf: retime, imtime: constructor from mesh added

- also on product.... (gf: re_im_time, refreq_imtime)
add get_component
gf: test of retime and refreq improved, the scalar_valued evaluator now works.
gf with products of meshes : bug corrected and commented
doc : gf, fourier,meshes