compute median family income as a ratio

This code computes the average median family income in a neighborhood by the following steps:

1. For each tract, compute `median_family_income_numer` as B19119_001E * B19119_001E, and computer `median_family_income_denom` as B00002_001E.
2. In the `crosswalk.rb`, the values from 1. are summed up to the neighborhood level using the tract-neighborhood crosswalk.
3. Divide the aggregated values from 2. to compute the neighborhood weighted averages.

Fixes #113