Users can add molecule viz state to model objects list

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If a user has visualized a molecule in their simulation called, for example, 
vol1, then a positions and shape/glyph object will also be created called 
mol_vol1 and mol_vol1_shape. These are mesh objects and can currently be added 
to the list of model objects. We should add a check to make sure that users 
cannot do this. 

Original issue reported on code.google.com by [email protected] on 5 May 2014 at 3:10