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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[sandro-elsweijer]

The folder should still be called t8_cad. It is the t8code module about cad. t8_cad_handle is just one part of it

[sandro-elsweijer]

There is probably a way to achieve the same thing with less functions. What we definitely need are:
constructor from file
constructor from shape
destructor
load from file
load from shape

[sandro-elsweijer]

Suggested change

	* Constructor of the cad shape handler.
	* Constructor of the cad shape handle.

[sandro-elsweijer]

A static member function means something different than a static function

[sandro-elsweijer]

what you probably want to use is private

[sandro-elsweijer]

Suggested change

	t8_debugf ("Deleting the cad_handler.\n");
	t8_debugf ("Deleting the cad_handle.\n");

[sandro-elsweijer]

Suggested change

	* Decrease the reference count of the cad handler.
	* Decrease the reference count of the cad handle.

[sandro-elsweijer]

Suggested change

	* Increase the reference count of the cad handler.
	* Increase the reference count of the cad handle.

[sandro-elsweijer]

Suggested change

	* Destructor of the cad shape handler.
	* Destructor of the cad shape handle.

[sandro-elsweijer]

Suggested change

	* If the reference count reaches zero, the cad handler is deleted.
	* If the reference count reaches zero, the cad handle is deleted.

[sandro-elsweijer]

Suggested change

	/** The reference count of the cad handler. TODO: Replace by shared_ptr when cmesh becomes a class. */
	/** The reference count of the cad handle. TODO: Replace by shared_ptr when cmesh becomes a class. */