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
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 linear_to_index to matsub_freq
Exposed MeshImFreq. Added field start_at_0 in ALL cases in hdf_write of mesh matsubara_freq. Otherwise, not same interface for case positive_only/not positive_only.
Added accessor positive only to MeshImFreq
Remove normalization from Hilbert trafo