dia to sml: fix empty graph test for graph builder
dia to sml: test scenario adding objects after graph is built
dia to sml: test scenario adding multiple objects to graph
dia: create simple mock factory for objects
dia to sml: add support for containers of objects in graph builder
dia to sml: test irrelevant object scenario in graph builder
dia to sml: use finish vertex graph builder spec
dia to sml: add remaining implementation of graph builder

- add support for child nodes and connections
- add orphanage and post processing
- tidy-up specs somewhat
utility: remove spurious brackets in IO for complex types
dia to sml: add test for first degree cycles in graph builder
dia to sml: add visit state and skeleton for visitor