update specs2 dependency
type-class based dataset conversions

- now used by all chart factories
  - allows to drop the explicit conversion step in most cases
- these new conversions are all provided in a selfless trait `DatasetConversions`
- internally uses `RichChartingCollections` for the actual conversion
- companion with identity factory ensures explicit conversions from `RichChartingCollections` can still be used
- on top, `RichChartingCollections` now provides everything for `GenTraversableOnce`, not just `Iterable`
  - `foldLeft` / `aggregate` are used instead of `foreach`