Dataswim

Feature release: update to support Altair 2 charts

Minor fixes and improvements:

- Fix Matplotlib import
- Add option to format date in linear regression chart
- Add methods to manage options and style faster

Maintenance release: fixes in Seaborn charts

Maintenance release: tests and fixes

Feature relase: add the `cvar` method to calculate the coefficient of variation

Maintenance release + improvements for maps and quantiles removal