In this pull request is a rough sketch of what we want the Quantity class to be able to do. It may not be complete, and there are a few thing we need to discuss:

1. How do we decide which units to return when performing operations between quantities?
2. Since 'in' is a reserved word in Python, and 'as' is as well (arg), we could do something like 'in_'? Does this seem awkward? Is there a better solution?