@alneberg please let me know if you think this should be a validation / verification.

For everyone doing library QC at NGI, the aggregate QC step will have

• A blue button "Fetch last known field"
• A step field "Fetch last known", with a drop-down selection specifying what to fetch
• A file slot, "Fetch last known field Log"

but it won't really affect anything unless it's manually run. And even if it's run accidentally, it should only affect the fields within the step, which can in turn be cleared, re-written and repopulated by running the regular aggregate QC scripts to fetch the field values from the QC steps.

Test steps

• A step in the middle of a workflow, with both inputs and outputs
• An aggregate QC step, with only inputs and no outputs

More test steps whose EPPs are impacted by the code changes

• Zika pooling
• ONT pooling
• ONT Update Amount
• ONT Calculate Volumes
• Calc from args

I ended up re-writing the function for recursive UDF fetching. This expanded the scope of the PR significantly, but I've attached tests on Stage that I feel cover the changes.

By my account, the only thing touching the accredited scope should be the Zika pooling.

Ready for re-review.