Listing all leaf types

[YooSunYoung]

Context

I removed a leaf type to remove unnecessary step in a workflow and then user (Christian in this case) had no idea which type to visualize/compute.

It'll be easier for users if there is an interface in sciline as well to list all the leaf nodes,
and then it can be used for visualization without any type specified,
and for workflow widgets.

Related Issues

• Should Pipeline.visualize default to show the full graph if no keys are given? #177
• [Requirement] UI components for selecting, configuring, and running workflows essreduce#28

Implementation Examples

There is an example already in essreduce.
https://github.com/scipp/essreduce/blob/aef4a3c1094dd15f1849aedc74e9960e1b922de3/src/ess/reduce/workflow.py#L52-L57

TODO

• Implement an interface that list all leaf nodes in the workflow
• Use the interface for visualize without any arguments
• Include the leaf nodes as examples in the KeyError or NoProviderFound error message.