WIP
Sync up and fix style
Use existing Plotly chart type: histogram2dcontour
Implement conversion for geom_density2d
Process the raw data, not the processed data
Fix merge conflicts (sync up with latest conversions)
Fix return of toBasic() for geom_density2d
Merge pull request #88 from ropensci/marianne-geom-density2d

Implement conversion for geom_density2d