Improve xy-functions

At this point the xy-functions should provide a decent set of functions
to handle xy data. A hand full of functions that work for "normal"
isis datasets are adapted to the xy data (e.g., plotting functions,
parameter save and load functions, ...)

Most of the things that work with 'fit_fun' should also work with
'xyfit_fun', the one major thing that is not implemented (and I also don't
see an easy way to do it neither a reason) is the handling of operator
functions.