Add a first implementation of HierarchicalFactFFT (along with ParamsFFT and a test which compiles).

- New baseline test (just a copy of basis HierarchicalFact except that we use child classes HierarchicalFactFFT and ParamsFFT). A real functional test will come later but we already know that Palm4MSAFFT is working so it shouldn't be difficult.
- Change Faust::HierarchicalFact::compute_errors() access modifier from private to protected (in order to override it later in FFT child), likewise all private fields move to protected access.
- New constructor in Palm4MSAFFT to initialize from a ParamsFFT object (like Palm4MSA can initialize from a Params object).
- Other minor changes (function decl. order, typo.).