Implement pyfaust wrapper FaustFactory.fact_hierarchical_fgft() and add a unit...

Implement pyfaust wrapper FaustFactory.fact_hierarchical_fgft() and add a unit test to validate equivalence to C++.

- getD() function added in Faust::HierarchicalFactFFT (for getting the diagonal matrix reduced to its diagonal vector -- opt).
- For the same reason added constructors into Faust::ParamsFFT (passing the diagonal vector only to define init_D).
- Refactoring in pyfaust.FaustFactory (adding function _prepare_hierarchical_fact())
- Refactoring in pyfaust.factparams.ParamsPalm4MSA (for the function _check_init_D_is_consistent())
- Added functions fact_hierarchical_gen() to handle generically the two algorithms fact_hierarchical() and fact_hierarchical_fft().