Document why we're typecasting some strings in SQL query results to numbers

[toolness]

In #326 we started typecasting some strings returned by SQL queries into numbers, but we didn't document why we were doing this. It would be nice to do that, to reduce confusion.

I think these might be why we're doing it but I'm not sure:

• In some cases, NYCDB is erroneously representing numbers as strings.
• JSON doesn't support 64-bit integers, whereas Postgres does (depending on the field type), so somewhere in the pipeline, something may be converting numbers into strings to avoid encoding issues.