Feat: Convert quads to triangles in trimesh conversion for non-pure quad/triangle meshes

[MartinSmeyer]

Describe your feature request

Currently, the bop writer fails for objects that contain mixed quad and triangular faces. This can be fixed by triangulating the quad faces.

Describe a possible solution

 def mesh_as_trimesh(self) -> Trimesh:
     """ Returns a trimesh.Trimesh instance of the MeshObject.

     :return: The object as trimesh.Trimesh.
     """

     # get mesh data
     mesh = self.get_mesh()
     
     # check if faces are pure tris or quads and triangulate quads if this is not the case
     if not all(len(f.vertices[:]) == len(mesh.polygons[0].vertices[:]) for f in mesh.polygons):
         # Triangulate quads
         self.select()
         bpy.ops.object.mode_set(mode='EDIT')
         bpy.ops.mesh.select_all(action='SELECT')
         bpy.ops.mesh.quads_convert_to_tris(quad_method='FIXED', ngon_method='BEAUTY') 
         bpy.ops.object.mode_set(mode='OBJECT')
         self.deselect()
     
     # get vertices 
     verts = np.array([[v.co[0], v.co[1], v.co[2]] for v in mesh.vertices])
     # get faces   
     faces = np.array([f.vertices[:] for f in mesh.polygons if len(f.vertices[:]) in [3, 4]])

     return Trimesh(vertices=verts, faces=faces)
     ```