arrays: details while writing doc

- add factories for matrices
- add make_immutablef from a lambda
- details
added mpi4py layer for experiments...

utility/mpi_mpi4py.py
is the new mpi.py to use with mpi4py.
For testing for the moment.
Passes on old solver ok.
arrays : renaming concept name

CuboidArray <-> Array
added make_cuboid_domain
Work on doc