Update VesuvioFittingTest

Now uses ComptonScatteringCountRate instead of basic CompositeFunction
to give a better overall fit.
Refs #7199
Update values for LoadVesuvio test.

The output is now a point data set in the difference case and the errors
were being calculated incorrectly before. Refs #7199
Merge branch 'bugfix/7199_vesuvio_fitting_diffs' into develop