Example DataFrame usage [Do not Merge]

As I was trying to understand TJ's numba code and fit it with what we had been talking about in terms of using DataFrames, I found it instrumental to actually refactor one function to try and spell out what it is that we're getting at ultimately.
Here's our dilemma: 
We like being able to reference (see [line 15](https://github.com/OpenSourcePolicyCenter/Tax-Calculator/compare/demo-df-usage?expand=1#diff-e8023e69480a2d8f863249f93791030bR15)) variables as attributes of a dataframe. At the same time, we would like to vectorize vanilla python statements as in line 15 and below instead of manipulating numpy arrays. To put it succicntly: we like referencing DataFrame columns but writing code logic for scalars that can then be vectorized.