Re-architect around pipelines

Removed loosely structured workflows. Now compose multistep work as pipeline modules, e.g. base.pipelines.segment, with the following blocks:

1. input parameters (Pydantic model)
2. output record (Pydantic model)
3. API mapping and docstring; also any special input validation that doesn't play well Pydantic, and special exception routing
4. pipeline logic function, which pushes results onto PipelineTrace

Refactored existing base and ilastik "workflows" into this pattern.