Moving decompose solids into CadToCsg class

[shimwell]

Description

Just looking at the attributes used in decompose solid it save a bit of data passing around if we move the decompose solid into the CadToCsg class. Save a few lines of code

The users will still not see this function as it has been made private, so it also won't appear in the docs. But I think this is the first step towards letting users perform the decomposition as a separate step and exposing this option to users

Checklist

• [x] I'm making a PR from a feature branch on my fork into GEOUNED-org/GEOUNED/dev branch
• [x] I have followed PEP8 style guide for Python or run a formatter such as black or ruff format on my code

[shimwell]

this one is ready for review if anyone has time

[shimwell]

Thanks for the additional changes, I've implemented as suggested