Consider creating a fake for GenericNeXusWorkflow

[SimonHeybrock]

Usage in a unit test:

def test_loki():
    wf = LokiWorkflow(nexus_workflow=FakeGenericNeXusWorkflow())
    wf.compute(IofQ[SampleRun])

This would allow for bypassing file I/O as well as dealing with large data (or having to create small files). The challenge is that unless instrument parameters are reasonable the workflow may fail or return data that is not verifiable, e.g., if all data is in unphysical regions.