Update several elements to finish the Palm4MSAFFT impl.

- Testing Palm4MSAFFT more properly in test_palm4MSAFFT.cpp.in with a special data set ./misc/data/mat/ref_test_PALM4SMA_FFT2.mat.
- Adding function get_D() to return the diagonal factor.
- Modifying Faust::prox_sp() to avoid trying to constrain the 0-norm to k if the matrix norm is already lower (because it does nothing more in that case).