Improve Chart.Grid

[kMutagene]

This PR drastically improves Chart.Grid, which previously only worked with 2D cartesian traces.

• Make Chart.Grid work for all trace types
• adapt docs
• add tests

It now works with ALL trace types as well as nested composite charts as long as they have the same trace type. here is an example:

[
    Chart.Point([1,2; 2,3])
    Chart.PointTernary([1,2,3; 2,3,4])
    Chart.Heatmap([[1; 2];[3; 4]], Showscale=false)
    Chart.Point3d([1,3,2])
    Chart.PointMapbox([1,2]) |> Chart.withMapbox(Mapbox.init(Style = StyleParam.MapboxStyle.OpenStreetMap))
    [
        let y =  [2.; 1.5; 5.; 1.5; 2.; 2.5; 2.1; 2.5; 1.5; 1.;2.; 1.5; 5.; 1.5; 3.; 2.5; 2.5; 1.5; 3.5; 1.]
        Chart.BoxPlot("y" ,y,Name="bin1",Jitter=0.1,Boxpoints=StyleParam.Boxpoints.All);
        Chart.BoxPlot("y'",y,Name="bin2",Jitter=0.1,Boxpoints=StyleParam.Boxpoints.All);
    ]
    |> Chart.combine
]
|> Chart.Grid(2,3)
|> Chart.withSize(1000,1000)
|> Chart.show

closes #163
closes #149