[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
gf: Work on documentation
gf: clean resize in operator =
gf: 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 ?
gf: 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
gf: 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.
gf: 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.
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).