[yt-dev] Field units from code to code

[j s oishi]

> While we're brining up Enzo-isms that we should get rid of, I think it might be a good idea to make a conceptual shift in the basic python UI.  Instead referring to the interface between the user and the data as a parameter file, I think instead we should be talking about datasets.  One would instantiate a dataset just like we do now with parameter files:
>
> ds = load(filename)
>

+1