Choice of fixed parameter is ignored in `pade_lstsq`

The implementation of `pade_lstsq` is incorrect, as the value of `fix_q` is ignored, see [lgtm](https://lgtm.com/projects/g/DerWeh/gftools/rev/pr-aff62894209556179736fd8f389178f19812e606).

The current heuristic, however, doesn't pass the tests. Which parameter is fixed played a significant role for the accuracy of the Pade-Fourier results.