arrays: length of domain corrected for &&
h5: add a parent in group

- add a parent in the group, to allow iteration on group
elements
- normally in h5, one needs to have the parent group and the name
of the group to iterate on its elements.
- added a parent (possibly empty) to get a simply syntax to
get the element of a group...
[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