mc_tools : improve treatment of infinite mc ratios

- when the ratio returned by an attempt of a move is infinite,
previous code was just throwing TRIQS_RUNTIME_ERROR.

- Now when the ratio is infinite, it is replaced by a large number
  (>1 is enough for metropolis), and the sign is properly updated
  using std::signbit.

- NB :

- a double/float in C++ can be :  normal/ zero/ nan/ infinite / subnormal.
  Here, the code will recover only from infinite case.

- std::signbit works for infinite (according to standard).
Fix #66: when mc_sign_type is complex

- for complex type sign, we do not do the previous check
(to be improved), since isinf, signbit are not defined for
complex numbers.
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
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.