Namespace of molecules and surface classes

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Currently, in CellBlender molecules and surface classes can have the same name. 
However, in MCell this is illegal, which makes sense since surface classes can 
appear in reactions. As such, a warning/error should also be generated in 
CellBlender when a user attempts to define a molecule with the same name as a 
surface class and vice versa. 

Original issue reported on code.google.com by [email protected] on 19 Mar 2014 at 8:01