Dynamic stats

Updates stats dynamically. This is how it works:

- When you click a feature, that feature is passed as an argument into the `update_stats()` function.
- `update_stats()` iterates over each amenity/stat in the list and does the following:
  - Skips `GEOID10` and `tract_name`
  - Checks to see if there are parks in the tract. If yes, sets value of `tract_amenity` to actual tract value. If no, sets value of tract_amenity to "N/A".
  - In line 73: Selects DOM element by amenity ID (the IDs match the amenity names), updates the HTML to include the stat value.

This PR also removes stats from popups.
1). Not sure where to add this comment- would it be easy to only display up to tenths decimal place for Total Park Acreage?

2). Tract 38.02 might have bad data? Southern part of tract area is clickable but northern area is not. It says there is 1 park but "Biggest Park" is blank.