ProtocolUnitResult should have a reference to the ProtocolUnit that created it

[IAlibay]

Downstream we often find ourselves in a situation where we need to at least know the type of the ProtocolUnit that created a ProtocolUnitResult. For example, this is important when we want to extract only the AnalysisUnit from a list of ProtocolUnitResults for further analysis.

It would be good if the ProtocolUnitResult either stored the ProtocolUnit or if it at least stored the type of that ProtocolUnit.

[dotsdl]

ProtocolUnitResult has a source_key field with this information.

[IAlibay]

@dotsdl the docstring for GufeKey's prefix method says "commonly indicates a classname", is classname guaranteed? If so, that might work for unit filterring.