Draft: Combine Backend Skeleton

[captainvogon]

Overview

This draft PR transitions the Combine integration from a standalone YAML parser to an integrated, object-oriented framework.

Key Changes

• dummy_fit.py: A conceptual user-facing script where physicists define their analysis (Datacard class) using Python members (channels, processes, systematics) instead of YAML.
• python/combine.py: The Combine backend wrapper. It mirrors the design of submit.py and batch.py by using importlib to load the user's fit script, instantiate their Datacard class, and validate the presence of minimal required members.

Next Steps

Once this skeleton is reviewed and approved, the next commits will migrate the existing string-alignment and datacard generation logic into combine.py, adapting it to read from the Datacard object rather than a YAML dictionary.