Implement Palm4MSAFFT::compute_grad_over_c().

- It needs to be checked again but it should work.
- It's based on CodeLuc/utils/grad_FFTgraph_S.m.
- The optimization is not complete (it just follows the opt. from Palm4MSA parent).
- Attribute D (matrix to diagonalize) added in Palm4MSAFFT.
- Other minor changes in faust_linear_algebra (comment complement).