address differences in variable availability over time #14
Description
Variables are not consistently available over time. For example, B25074_051 (household income by gross rent...) is not available pre-2014, at least not by the same variable name. Auto-documentation should create a variable dependency object. Then, compile_acs_data should read this variable dependency and compare variable availability for each requested year. When a variable is not available for some or all years, it should be included with all NAs, and this should be reflected in the metadata and returned to the user as a warning.