arrays: a forgotten std::forward in lazy evaluations...

A(i_,0) was not compiling due to a missing forward ...
Fix a bug in bosonic Fourier transform

Appears for full_bins and bosonic GFs.
Add documentation for web profiling
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.
gf: correct auto_assign

- g(om_)(i_,j_) was not working.
- add the triqs_clef_auto_assign function for matrix_proxy.
- simplified a bit the triqs_clef_auto_assign for gf.
arrays: add lazy call to matrix_tensor_proxy

- correction to previous patch
clef : correction of TRIQS_CLEF_IMPLEMENT_LAZY_METHOD for clang 3.3

- there was a compilation pb with clang 3.3
- replace the decltype (with this !) by an explicit type
correction: previous commit, forgot a namespace

- in some case, need to add triqs::clef in the macros.
(was not compiling doc).
arrays: fix fold and update documentation

- fold was not correct in e.g. passing an int as init instead of a
  double (was leading to narrowing in return).
- better return type deduction.
- there was an error in the doc (order of argument in the lambda !)
- add a more complex example (Frobenius norm of matrices).
Added ising2d test for statistics.
Statistical tools documentation.
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.
statistics : fix get_value_type detection

- clean a bit the auto_correlation_time