allow only_built to be set to false
two small fixes

make the add_columns function into an injectable instead of a function defined in this module

add the ability to pass specific forms to feasibility for testing so that you can just check residential feasibility and ignore the others
Merge branch 'master' into subsidized-dev
don't cache total residential units
Merge pull request #5 from synthicity/subsidized-dev

rename some functions, un-cache some variables, and pass the only_built parameter through utils to the developer model