Add a mouse wheel mixin
Improve css for the drill down
Add possibility off drill down and roll up on regions
Add possibility to set the number of zoom levels

By default, the number of zoom levels is 0 and mouse scroll won't do anything.
And you can set the number of zoom levels with setNbZoomLevels to enable the possibility to zoom with mouse wheel up to the defined number of levels.
Prevent drill across
Adapt geo choropleth to wheel mixin
[wip] Test choropleth chart