CorrelationDistance can return non-negative distances

[joseamuniz]

With gryffindor-sliderin.csv, add the following assertion to CorrelationDistance's apply method:

if (1.1 - abs(correlation(gradesList)) < 0 )
throw new RuntimeException("Computed negative distance") ;

The distance between Hagrid and Snape is negative due to a strong negative correlation of -1.53.

[jchernan]

In method CorrelationDistance.apply loop through list versions of getStudents and getAssignments instead of sets to avoid duplicated grades from being removed.