[Developers] Updating /chemistry/chemical_element/atomic_mass expected type

[Bryan Cheung]

http://lists.freebase.com/pipermail/data-modeling/2009-April/001615.html
https://bugs.freebase.com/browse/DA-636

The proposed plan is to add another atomic mass property that links to  
the /chemistry/atomic_mass cvt, migrate the current values from / 
chemistry/chemical_element/atomic_mass to the new property, then  
delete the old property.  Since we're expanding the property from a  
simple link to a cvt, queries that are expecting a simple link will  
break.  I would like to start making this change - does anyone need  
more time to adjust their queries?