Ability to add multiple scorers to generator

[ap-buf]

I was at one point using the Simple Generic Generator similar to this example: https://github.com/Kitware/nrtk/blob/main/examples/simple_generic_generator.ipynb
to generate some metrics

I wish that you could add multiple scorers to the generator so there was a clean concise way to get many metrics from a batch of perturbation values. The scorer interface has a strict return type so it can't be finagled to return many results.