modified xyfit to use dataset-metadata instead of a global variable

- xy-data, -model and additional xyfit_metadata are now separated from each other

- changed interface for xy-fit-functions,
  such that they can now only modify the model's x- and y-arrays,
  but not have other side effects

- `change_xy_global_data' has been renamed to `set_xyfit_qualifier'
  TODO: The xyfit_qualifiers should be specific to the xy-fit-function
        and not to the xy-dataset.

- load_xydata has been, as also suggested by Moritz Böck, removed