add mechanism to create new variables

[miserman]

To allow for more custom data analysis and potentially reduce the size of loaded data (by not needing to include calculable variables), it could be nice to have a means of adding new variables, calculated from existing one.

Use Cases

• Create totals from broken-out categories (e.g., total population from separate demographics variables).
• Create percent variants from count categories (maybe using the created total).
• Allow specification of custom variables (e.g., a female to male ratio from given female and male count variables).

Implementation

• The main calculation mechanism should probably be in the DataHandler.
• Could have at least 3 interfaces (where the new variable can be specified): R, browser UI, URL parameter.
• For category-based variables, probably allow some sort of specification through measure info.
• For custom variables, could be very free-form, which might involve some custom parsing (e.g., female_ratio = female / male, where variable names and operators would need to be recognized), or could be limited to a few pre-defined functions (e.g., a ratio function, with accepted inputs of two existing variables).
• Might want a means of also specifying measure info for the new variable.

Stipulations

• Would need to be treated as a regular variable throughout.
• If new variables are to be available through the API, would need a means of passing custom specifications through the URL.
• Will probably want new variables to be stored in local storage when specified through the browser.