Append existing subcomponent to Component.features

[djross22]

If you try to append the same subcomponent to a component's features attribute, it throws an error (which I think it should) - but it also adds the subcomponent to the features list anyway.

Example code:

>>> ptet = sbol3.Component('pTetR', sbol3.SBO_DNA)
>>> circuit = sbol3.Component('circuit', sbol3.SBO_DNA)
>>> ptet_sc = sbol3.SubComponent(ptet)

>>> circuit.features += [ptet_sc]
>>> circuit.features += [ptet_sc]
throws an error, but circuit.features still has two copies of ptet_sc:
>>> circuit.features
[<sbol3.subcomponent.SubComponent object at 0x0000027E36E6F220>, <sbol3.subcomponent.SubComponent object at 0x0000027E36E6F220>]

[tcmitchell]

It sounds like whatever condition is causing the error needs to be checked prior to adding the child object. Thanks for reporting this bug.