Correction: left some C++14 used in debug...

- decltype(auto) was left behind in 2 places...
gf: correct evaluator for gf imfreq (remove mesh_pt)

mesh_pt should NOT be done directly by (),
it should be casted to matsubara_freq.
Removed an old line of code which we forgot to clean
when introducing matsubara_freq
mc_tools : add access # proposed/accepted. Add debug_stream

- debug_stream : conditional ostream with a lambda condition for debug.
Added draft of doc
Added ising2d test for statistics...
Fixing mistake: was taking next-nearest-neighbors...
Further work on ising2d test. Added output.
Statistical tools documentation, c'ed.
Added constructor of observable on binned_series