feature/graphical fixes

[rhayes777]

Resolves #1110

[Jammy2211]

I ran the following integration test:

https://github.com/Jammy2211/autolens_workspace_test/blob/main/slam_graph/integration/base.py

For the most part, this works, however the model.graph file is now output in files as a .json file, which is fine but it was also a text file before which gave the users information, so keeping it also as a text file next to model.info would be good:

Although maybe just keeping it as model_graph.json is fine, but at the moment the format is very human unreadable:

[Jammy2211]

Nevertheless I will merge this as itg'll help me test graphical models.