moving decompose solid into 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 not see this function as it has been made private, so no changes to the user workflow. But I think this is the first step towards letting users perform the decomposition as a separate step

Checklist

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