Friend functions do not produce doc from c++2rst

[HugoStrand]

Dear all,

Building the doc currently produces a lot of warnings of missing functions, due to missing content that should be generated by c++2py. In particular this seems to be the case for friend functions, e.g. these functions for atom_diag.

/Users/hstrand/dev/triqs/cbuild_master/doc/cpp2rst_generated/triqs/atom_diag/atom_diag.rst:147: WARNING: toctree contains reference to nonexisting document u'cpp2rst_generated/triqs/atom_diag/atom_diag/atom_diag_worker'
/Users/hstrand/dev/triqs/cbuild_master/doc/cpp2rst_generated/triqs/atom_diag/atom_diag.rst:147: WARNING: toctree contains reference to nonexisting document u'cpp2rst_generated/triqs/atom_diag/atom_diag/operator<<'
/Users/hstrand/dev/triqs/cbuild_master/doc/cpp2rst_generated/triqs/atom_diag/atom_diag.rst:147: WARNING: toctree contains reference to nonexisting document u'cpp2rst_generated/triqs/atom_diag/atom_diag/h5_write'
/Users/hstrand/dev/triqs/cbuild_master/doc/cpp2rst_generated/triqs/atom_diag/atom_diag.rst:147: WARNING: toctree contains reference to nonexisting document u'cpp2rst_generated/triqs/atom_diag/atom_diag/h5_read'

I can not make c++2rst generate docs for these functions. Olivier, can you have a look?

Best, Hugo