Thousands separator for to_csv
Pandas exposes a `thousands` optional parameter to `read_csv` used to specify a custom thousands separator, so that `1,000` or `1_000` can be successfully parsed to a numeral in the resulting DataFrame.

Unfortunately, Pandas is missing the very same parameter for `to_csv`, so that a DataFrame containing `1000` ends up serialized to csv as `1,000` or `1_000`.

I understand the general issue of custom float formatting in Pandas remains open (see https://github.com/pandas-dev/pandas/issues/4668) and may not even find a solution. However, this particular use case sounds a bit more accessible, since it has been done successfully for `read_csv`.