Simulation refactoring and operator graph work

What does this MR do and why?

This makes some substantial refactoring the backend simulation and solver code to generalise it to other simulation types.

It also adds in some initial attempts at graph/symbolic solving.

We decided to merge this in and see how things fair for now and attempt plane wave polarisation models with it as is.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Includes tests for added/modified code
• Added or modified the documentation if relevant
• Docstrings added/modified for new/modified functions
• Committer/reviewer has ran the documentation stage when ready to merge