Move t8_cmesh_set_tree_vertices

[sandro-elsweijer]

Describe your changes here:
The t8_cmesh_set_tree_vertices function was in the t8_cmesh_vtk_writer.h header and it made no sense. Based on a Mattermost discussion the according changes were made. There is no new code in this PR, I only moved things around.

Since the vertices of a tree are only needed, if a t8_geometry_w_vertices geometry is used, the function is now part of this header. Therefore, it is automatically loaded when a t8_geometry_w_vertices or derived geometry is used. This should also enshure, that all external applications which link against t8code do not have to be adapted (apart from that in some cases the t8_cmesh_vtk_writer.h header does not have to be included anymore).

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[jmark]

@sandro-elsweijer What is the meaning of _w_ in all these names? Is it an abreviation for _with_ or anything else? I think it wouldn't hurt to spell it out. This makes the intent of the code much clearer.

[sandro-elsweijer]

@sandro-elsweijer What is the meaning of _w_ in all these names? Is it an abreviation for _with_ or anything else? I think it wouldn't hurt to spell it out. This makes the intent of the code much clearer.

Yes you are right, this can be confusing. Changed that