Revise fit examples

Python scripts in Examples/fit* have not been touched during recent refactoring of other examples. So they need to be brought in line.

- [ ] Process command-line arguments with ba.parse_args. not used anymore

• Standardize script structure; use functions from ba_plot. - good enough for the time being
• Put them under CTest coverage.
• Check convergence. There is no convergence at least for fit/specular/PolarizedSpinAsymmetryFit and fit/specular/Pt_layer_fit - has been repaired in the meantime