Worried by Source->Output registering

[John Biddiscombe]

Source->SetOutput(dataobject) causes output to be registered, and output in
turn registers source.

Problem is that if Source->Delete is called, it hangs aound until the
dataobject is destroyed. This can consume excess memory in some (complex
filter) cases.

Should there not be a 

Source->UnregisterAllOutputs()
function for when we wish to delete a filter and make it go away for real

[echoes of Queen Elizabeth talking to Blackadder saying "When I throw
something away I don't want it to come back", on the subject of Boomerangs]

JB
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