Use the Pint library for units

[Castronova]

Represent model units using Pint.

http://pint.readthedocs.org/en/0.6/tutorial.html

[Castronova]

Need to implement unit conversions during data transfer between models.

• Add Unit conversion button to Link Creation window
• Use Pint to perform unit conversions
• Add data transform button to Link creation. This should allow us to aggregate, disaggregate , etc... , data on the fly.