OpenFisca conventions

[nikhilwoodruff]

We've got the first style guidance in the wiki now, and I think we should also have some guidance/conventions around the OpenFisca framework and writing policy logic. Here are a few ideas, I thought I'd put them here to see if people agree/disagree and have ideas for more.

• Parameters should share a file only if they'd share the filename (indexable e.g. by state parameters are fine to be grouped)
• The label attribute should not be more than a few words; use the documentation attribute for anything longer
• Avoid overly long formulas: splitting up is good for readability, testing, unimplemented features for web apps (like explanations of variable outputs). Code duplication not a huge performance concern as much of the function calls/parameter lookups are cached.

@MaxGhenis @nmrodelo @moon-home

[nikhilwoodruff]

Convention: assigning to variables is preferable when code formatters split bracketed variable calls over multiple lines ( from PolicyEngine/policyengine-uk#416 (comment)) (@MaxGhenis )