Make workflow.py obsolete for standard cases

[danielhuppmann]

Most of the repository with a workflow.py file are very generic, see for example https://github.com/iiasa/prisma-workflow/blob/main/workflow.py.

This could be abstracted away by moving this into nomenclature.yaml, for example

workflow:
    [definitions: "some-custom-definition-folder"]
    processors:
      - region-processor: "mappings"
      - [... ]

Not sure what the ideal entrypoint would be (from the scenario-processing side), maybe a new nomenclature-module that can be called if workflow.py doesn't exist?

[phackstock]

It's a good idea for sure and one that we should definitely implement in the future. For now, though I'd see it as a low priority item since it would not save us a huge amount of time or pain.