Modify GivensFGFT::get_D() with opt. bool arg. to make possible to get ordered D (in ascendant order of eigenvalues along the diagonal).

- Adding also GivensFGFT::get_ord_indices() to get the ordered indices. It's useful for getting the corresponding columns of the Fourier matrix.
- Deleting the sorting part of the test GivensFGFT.cpp.in, because now we can directly rely on get_D(true) and get_ord_indices().