Allow non-numeric rubric and metric outputs

[levon003]

The requirement for numeric outputs is somewhat arbitrary. It would be nice to support free-form or categorical outputs as well.

• We would probably need to label the type of output in the MetricItem.
• We would have to check the output_type of the dependent MetricItem before doing numeric range comparisons.
• We would want to add a way to do equality comparisons, not just range comparisons against min_value/max_value

In issues.txt, we had previously written "maybe min_value and max_value are both optional because perhaps downstream calculation just dependencies on the value itself (or the metric being run), rather than some threshold?" This issue is related, as it involves adding equality comparisons.