adds the default formula to the columns needed for to_frame

also removing a small warning in sqft pro forma
deal with the case where model_expression is None
pep8
Merge pull request #82 from synthicity/add-default-formula

adds the default formula to the columns needed for to_frame
Ignore NaturalNameWarnings from pytables

They come up when we store data under keys like '2010/buildings'
and warn that attribute access won't work (but key access works fine).
We don't care about this warning.
Merge pull request #81 from synthicity/save-data

Add the ability to auto-save tables during simulations
Merge branch 'master' into change-to-job-spaces