Create separate [ S * O ] index class for case where there is only one predicate

[pstutz]

This would save the overhead of storing a full reference for the one integer.

[pstutz]

Thanks a lot to @kolarm for the suggestion.

[pstutz]

If adding/removing a vertex should create issues, then always having the Int on the class would still be more efficient than paying for the boxing, given the huge number of instances that have only 1 predicate.