pytriqs: add option to use mpi4py instead of boost.python.mpi

- to choose mpi4py over boost.python.mpi, pass -DPython_use_mpi4py=ON

- boost.mpi stays the default for python, but
  adding the possibility of using mpi4py instead.

- Due to numerous pbs on recent OS X + brew installation.
  boost.mpi does not work (crash in openmpi ??), mpi4py does

- the pytriqs/utility/mpi.py is now configured by the cmake (like
  version.py) to prepare one of the two possibilities.

- TODO:  not tested carefully : to be done in other machines.
arrays: fix mapped_function (conj overload)

- conj overload should return complex, as in standard.
- changed the test output for mapped_function.
- fixed make_matrix_view and example in doc.
work on doc for gf, and details.

- little details : code cleaning, clang formatting,  along
with documentation writing for c++ gf.
- separated the mesh in small class for better doc.
- work on documentation : reorganize specialisation, ...