robustFit performance improvements

Switching from Statistics.Function.sortBy to List.sortBy allowed taking advantage of the laziness of lists.
Switching other parts of the code to use lists further improved performance.
All in all about 6 fold increase in performance (down from 18 sec. to 3 sec. on my poor machine) for the relevant test case.
Further improvement might be achieved by switching to lists altogether but that incurs significant interface changes and thus probably not worth the effort.