Measurement Units

The ItemFamily in TinkerCell has now been modified to include a Pair called measurementUnit.

This pair indicates the unit by which a part is measured. The measurement unit for Species family, for example, would be ("concentration", "mM"). 

This means that values such as "concentration" that were once places under "Attributes" are no longer attributes. The "Initial Value" data stores these values. The name "concentration" can be obtained by looking at the measurementUnit of the Species family. The same applies to all other families. Connection families do not have any measurement units at the moments (should they?). 

Important implications:

1. units are not determined by the model, but by the family tree
   
2. users cannot change the units while constructing models
3. changing units means changing the parts/connections tree
4. units are part of the "standards" defined by the parts and connections tree