Fix #142. boost serialization of matsubara_freq_mesh.

boost serialization of matsubara_freq_mesh was broken
by the introduction of the window for the MPI api...
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
[gfs] Rework evalutor for multivar gf
[gfs] clean test, introduce common, ...

- clean bz1
[gfs] Add no_cast for the mesh_point evaluation.

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

- make_regular (x) -> return a regular copy, forcing the evaluation
  of expression templates.
- TODO : do it for array_expr, gf_expr
[gfs] mesh product: index_t as a class

- to permit construction from a tuple of mesh points to components
[gfs] 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).
[gfs] 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