Add 3D scatter plot

[crackcomm]

It allows us to create scatter plots in 3D. It works for matplotlib >= 3.2.0, for older versions it's required to import mpl_toolkits.mplot3d.

let fig = Fig.create () in
let ax = Fig.add_subplot_3d fig ~nrows:1 ~ncols:1 ~index:1 in
Ax3d.set_xlabel ax "max drawback";
Ax3d.set_ylabel ax "loss";
Ax3d.set_zlabel ax "cost";
Ax3d.grid ax true;
Ax3d.scatter ax ~c:Yellow values;

I created Ax3d.t because otherwise it could be possible to plot 2D values on it while generating runtime warnings.

[LaurentMazare]

Thanks for the PR!