Fix #20
[TO BE MERGED] Expr template for gf

  modified:   triqs/gfs/data_proxies.hpp
  modified:   triqs/gfs/gf.hpp
  modified:   triqs/gfs/gf_expr.hpp
[API change] gf : factories -> constructors

- Make more general constructors for the gf.
  gf( mesh, target_shape_t)
- remove the old make_gf for the basic gf.
- 2 var non generic gf removed.
- clean evaluator
- add tensor_valued
- add a simple vertex test.
- clean specialisation
- Fix bug introduced in 1906dc3
- forgot to resize the gf in new version of operator =
- Fix make_singularity in gf.hpp

- clean resize in operator =

- update h5 read/write for block gf
  - changed a bit the general trait to save *all* the gf.
  - allows a more general specialization, then a correct for blocks

- NOT FINISHED : need to save the block indice for python.
  How to reread ?
  Currently it read the blocks names and reconstitute the mesh from it.
  Is it sufficient ?

- clean block constructors

 - block constructors simplest possible : an int for the number of blocks
 - rest in free factories.
 - fixed the generic constructor from GfType for the regular type :
   only enable iif GfType is ImmutableGreenFunction

- multivar. fix linear index in C, and h5 format

  - linear index now correctly flatten in C mode
    (was in fortran mode), using a simple reverse of the tuple in the folding.
  - fix the h5 read write of the multivar fonctions
   in order to write an array on dimension # variables + dim_target
   i.e. without flattening the indices of the meshes.
   Easier for later data analysis, e.g. in Python.

- merge matrix/tensor_valued. improve factories

  - matrix_valued now = tensor_valued<2>
    (simplifies generic code for h5).
  - factories_one_var -> factories : this is the generic case ...
    only a few specialization, code is simpler.

- clef expression call with rvalue for *this
- generalize matrix_proxy to tensor and clean
Update documentation

Changelog and some updates were done.
clean exception catch in tests

- exception catching catch in need in test
because the silly OS X does not print anything, just "exception occurred".
Very convenient for the developer...
- BUT, one MUST add return 1, or the make test
will *pass* !!
- --> systematically replace the catch by a macro TRIQS_CATCH_AND_ABORT
 which return a non zero error code.
- exception : curry_and_fourier which does not work at this stage
(mesh incompatible).
gf: clean draft of gf 2 times

- also comment the python interface for the moment.
- rm useless tests
gf: Fix out of bound in linear mesh in gf eval.

- When reaching the last point of the grid,
move one step back and set w =1.
- fix the gf_2times but and related issues.
clean the TRIQS_COMPILER macros

- from now on, C++11 compilers only.
- no ressources to maintain old stuff.
gf: Work on documentation
remove c++11 tests
implement array_const_view
implement gf_const_view