gf. add __call__(double) to gf_imtime, wrapped from C++ evaluator
rng : preview
add nfft detection
Correction to previous commit
clef: first version of generic sum function

- to improve with in the case where function return an expression
  template, not a regular type, with a make_regular function
clef: fix lazy method call for gcc

- was not compiling on gcc (forward declaration issue).
- now clearer
- add a trivial print change in the mc class (to avoid trivial commit).
clef: add version c14

- a slightly different version for c++14,
to avoid some decltype and have clearer error messages.
only in c++1y mode.
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
Made autocorrtime calculation from binning faster.

Bin size is doubled at each step to converge faster and reuse previous binned series.