Added test for statistics + fix bug variance

This is the basic example of the paper.
Also fixed a strange problem in computation of the variance:

I have identified cases where:

$$ 1/N sum_i (x_i- m)^2  \neq 1/N sum_i x_i^2 - m^2 $$

with $ m = 1/N sum_i x_i $

In particular, the RHS (which corresponds to previous implementation) may give **negative** variances!! LHS seems to be more accurate (especially in cases where x_i is very close to m) --> changed implementation to LHS
[doc] update to preliminary introduction, prettification
[doc] Fixed example #99
[doc] stated faqs.rst with text file printing info
[doc] faq: added more about text file saving
rm parameter class, fix #124

- now replaced by simpler and better code generation solution
- TODO: write the documentation, implemented for cthyb solver
[wrapper] Split the converters code, fix #121 and hello_world

- The wrapper_tools.hpp is split into several converters/xxx.hpp
- to include only on demand.
- This fixes the hello_world from the paper
Fix #145 : Compilation on Ubuntu 14.04 32 bits

- one integer type was wrong
- detection of python : patch is different for 32 bits than the usual path in 64 bits.
Fix #152
[gfs] Fix out of bound error in fit_tail

- n_max must be checked against max size of mesh
  or bad input in python will lead to undefined behaviour,
  random results and valgrind error....

- At this stage, I leave the ENFORCE_CHECK macro in fit_tail.cpp
  for checks.
  TO BE REMOVED when code is reviewed.
Silence some gcc warnings.
[gfs] Opt->Evaluator.  Rework evaluator for multivar gf

- fix on_mesh :
   method can be called with mesh_point
   on_mesh wrapper class can be called with lazy arguments.

- add make_regular function to fix sum
  - make_regular (x) -> return a regular copy, forcing the evaluation of expression templates.
  - TODO : do it for array_expr, gf_expr

- mesh product: index_t as a class
  to permit construction from a tuple of mesh points to components

- Opt -> Evaluator
  - Opt template param is replace by Evaluator
  - Evaluator : default is void, (default evaluator as before)
  - allows to add a custom evaluator (cf new test sym3).

- Change evaluator concept
  - Now evaluator must have one constructor taking (G* g)
    with the gf, given at construction.
  - Allow to prepare some data.
  - E.g. sym3
[gfs] Test mpi for multivar gf.

TODO : more test.
Fix hdf5 formats

- block gf : clean the hdf5 format
- doc