spatial interpolation functions

[cole-brokamp]

• these functions will use {{cincy}} weights
• include vignette with example of areal interpolation

[cole-brokamp]

previous comparison we did of census tract dep_index interpolated to ZCTAs using area-weighted interpolation versus simple averaging interpolation

should we include this in a vignette? are all of our interpolations going to be area weighted, or should we also do population weighted averages?