Fixing column chart axis label rounding errors

A change we made in the soon-to-be-open sourced (the next few days, it'll be in https://github.com/washingtonpost/datawrapper) datawrapper fork, that addresses rounding errors on column chart axes.

There's some inconsistency between how column chart and grouped-column chart
is structred, looking at the `horzGrid` is just one example (compare how `theme` is defined),
`__gridlabels` vs `tickLabels`, etc.)

The fundamental challenge here was parsing out when `formatValue` in dw.js/src/dw.chart.js was used (which accepts a boolean rounding arg), and when `formatValue` defined in the plug was used (which just overrides the former func and replaces it with `columnFormatter` in `/dw.js/dw-2.0.js`)