Round all floats for frontend

[mattbowen-usds]

While trying to understand what drives the size of our pbf tiles, we determined that adding an unrounded float that exists in most tracts increases file sizes for zoom level 5 by about 20% over the minimum file size possible.

We looked, and all of the float fields in the tiles are acceptable to round to two digits of accuracy, so make that change.

[mattbowen-usds]

I did a little extra analysis, and here's WHY that matters:

• the pbf format specifies key-value pairs (the variables we send for every tract) have to be string:string. The type information is sent separately, but in the file everything is a string
• The format extracts strings into a "string table," which is why booleans and state names are so inexpensive --- every unique string goes in the string table exactly once
• This also explains the difference between rounded and unrounded floats --- rounded floats add at most 100 strings to the string table (0.0 - 1.0), but unrounded floats can add a value for every tract (which is like 14K for some of the bigger files)

If I were building this again, I might consider putting much less data in each tile (basically the minimum necessary to render the map's styles and lookup the tract) and then add a separate call to get the rest of the fields via ajax (and maybe return a bunch of tracts at a time by grouping the results by FIPS code) and maybe even use a more compact encoding like messagepack. But hindsight is of course 20:20.

[mattbowen-usds]

This ended up saving 448KB off our worst-case tile, which isn't nothing but also isn't anything I'd write home about (about 10%)