Simple UML class diagram

[PeterParslow]

See #24 (comment)
"I think the informal pictorial UML class relationship might help address this. The class diagram should be added to the document" @6footdestiny : I was thinking today that the simple UML overview of GEMINI would help
I think you mean a diagram from https://github.com/agiorguk/gemini/blob/main/resources/GEMINI%20UML.eapx - but I no longer have an EA license to open it. Could someone export a relevant diagram in a format good for embedding in perhaps https://github.com/agiorguk/gemini/blob/main/docs/1037-uk-gemini-standard-and-inspire-implementing-rules.asciidoc