Add a sort values parameter to assert_xy

[nkorinek]

Sometimes, in xy plots, there's no consistent way to test that the lines are exactly the same. For example, if a student is plotting two lines, the xy plot data grabbed from the plot will change based on which line they plot first. For plots like this, although it's not ideal, it's more consistent to sort the xy values grabbed from the plot and grade against the sorted values. This way we check that all of the values from all lines are in the plot consistently.