Update HierarchicalFactFFT, Palm4MSAFFT and their tests.

- Adding a relevant test for HierarchicalFactFFT
- Replacing in HierarchicalFact the attribute object palm_global by an object pointer in order to implement polymorhism in HierarchicalFact (which uses Palm4MSA) and HierarchicalFactFFT (which uses Palm4MSAFFT). A destructor is here to delete the object from heap.
- Adding cmp test for each factor (of Faust approximation) in test_palm4MSAFFT.cpp.in (Relative diff. on Uhat each factor is not greater than 10^-4 comparatively to matlab results).
- Fixing mem. leak (data buffer) in Faust::Palm4MSAFFT::compute_D().
- Correcting error of D gradient computation in compute_D_grad_over_c().
- Implementing missing Faust::Palm4MSAFFT::compute_c() with empty_function because Palm4MSAFFT has a constant step size.
- Forcing ParamsPalmFFT.constant_step_size to be always true (as in matlab script).