Add support to "name" `Pipeline` stages and reference them by name

## Motivation
Different stages of a pipeline may have meaningful names associated with them, and logic from within or outside the pipeline may want to access a certain stage's signal by name rather than absolute index. This helps make refactoring pipelines even easier, since adding a stage will not modify stage names.

## Desired solution
Add to or extend functionality of `Pipeline`s to support naming stages.

## Additional details
Some thought has to be put into where the name goes and what the type of the name should be. Should it always be a `String` or should it be parameterized so that an `enum` could be used? The current implementation is a `List Function(PipelineStageInfo p)>`, so modifying the input to `Pipeline`s to accept some new `List` or a `Map` would be a significant API change, but maybe it is appropriate so that more functionality can be more easily added in the future.