Restructure AiidaWorkgraph to builder class or inherit from core workflow

[agoscinski]

It does not really make sense that this is a class as it just converts it to a workgraph. There is no internal state that is abstracted out.

Change to WorkgraphConverter WorkgraphBuilder with a staticmethod to create a workgraph from a core workflow or inherit from the core workflow and just add a to convert the workflow. Will not do this in #45 since rebasing afterwards would be unnecessary work. I think the former is a bit more flexible for the tests.