Improvement: Move CAD management to t8_cad_handle with cmesh ownership

[LenaRadmer]

Describe your changes here:

Changing t8_cad to t8_cad_handle for a more explicit CAD shape management. The cmesh now owns the CAD handle and uses a reference counter to manage it.
Also, the t8_geom prefix was removed for everything related to CAD shape management for cleanup. The mesh reader and geometry implementations were updated to support this new handle system.

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