handle non-numeric indicators

In the ETL script it's assumed that indicators are numeric data (link to [code]). However that is not always true. We should accept non-numeric indicators instead of raising Error.

[code]:  https://github.com/open-numbers/ddf--gapminder--fasttrack/blob/8aa2569102d90dc933fe6837ec52d24cf70b08eb/etl/scripts/etl.py#L110