Calling child algorithms with workspace groups doesn't populate output workspace property

[rbauststfc]

If you run an algorithm on a workspace using the algorithm manager then it is not possible to retrieve the output from the "OutputWorkspace" property, it is instead put into the ADS:

input_ws = LoadNexus("INTER45455")
alg = AlgorithmManager.createUnmanaged("ReflectometryBackgroundSubtraction")
alg.initialize()
alg.setProperty("InputWorkspace", "input_ws")
alg.setProperty("ProcessingInstructions", "100-145")
alg.setProperty("OutputWorkspace", "result")
alg.execute()

output = alg.getProperty("OutputWorkspace").value
print(type(output)) # Gives None type and the workspace is instead stored in the ADS

However, if the above is run as a child algorithm, then you can retrieve the output directly from the "OutputWorkspace" property:

input_ws = LoadNexus("INTER45455")
alg = AlgorithmManager.createUnmanaged("ReflectometryBackgroundSubtraction")
alg.initialize()
alg.setChild(True)
alg.setAlwaysStoreInADS(False)
alg.setProperty("InputWorkspace", "input_ws")
alg.setProperty("ProcessingInstructions", "100-145")
alg.setProperty("OutputWorkspace", "result")
alg.execute()

output = alg.getProperty("OutputWorkspace").value
print(type(output)) # Gives a workspace

This makes it easy to retrieve outputs without having to interact with the ADS when calling algorithms from within other algorithms.

If an algorithm is run as a child against a workspace group, though, then the output always seems to be stored in the ADS and cannot be retrieved from the "OutputWorkspace" property:

input_ws_grp = LoadNexus("POLREF32130")
alg = AlgorithmManager.createUnmanaged("ReflectometryBackgroundSubtraction")
alg.initialize()
alg.setChild(True)
alg.setAlwaysStoreInADS(False)
alg.setProperty("InputWorkspace", "input_ws_grp")
alg.setProperty("ProcessingInstructions", "520-640")
alg.setProperty("OutputWorkspace", "result_grp")
alg.execute()

output = alg.getProperty("OutputWorkspace").value
print(type(output))  # Gives a None type and the workspace is instead stored in the ADS

If an algorithm is being run as a child then, regardless of whether it's been passed a single workspace or a workspace group, I think ideally it should return it's output via the "OutputWorkspace" property. Among other things, this would prevent unexpected behaviour when running algorithms as a child with a workspace group.

I've marked this as an investigation for now as I think we probably still need to determine if this is a bug or a conscious design decision (maybe due to technical obstacles?).