Fix 1: Atomic structures based on meta spheres were not visible because the meta balls were hidden by default, T62804
Fix 2: Option 'Mesh UV spheres' for atomic structures lead to an error message, T62804

Comments
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1: It i strange that a representative mesh sphere or NURBS sphere in a dupliverts structure
can be hidden, which is not the case for meta balls. A representative meta ball needs to
be 'visible' and not hidden. By default this is now the case.
2: The 'size' option in 'bpy.ops.mesh.primitive_uv_sphere_add' does not exist anymore.
Option 'size' was removed.