Support FCDO subscriptions #110
Description
Currently our subscriptions only support FCDAs.
If ExtRef elements are used for ReportControl blocks then it would be useful to be able to support FCDOs.
I suspect it might be best to do this with a separate function rather than overloading our existing functions.
This would also help for supporting older files idn e.g. the subscriber plugin. One reason why as a utility we couldn't adopt that immediately would be lack of support for FCDOs which were (perhaps incorrectly) often used for GOOSE with some suppliers.
(I'm not sure whether this is a good idea for Milestone 1.0.0, I think it would be more important to do the other items listed).