[Chart] Contour Plots

[Sacha Sokoloski]

Okay, so I also just forked 0.17 and added in the contour plotting, as 
well as a test script. It works pretty well, but the primary problem at 
the moment is that the lines should really be drawn as bezier splines in 
order to smooth the appearance of the sampling. Graphics.Rendering.Cairo 
exports the function 'curveTo' for rendering bezier splines, so it 
should be easy to add, however I wouldn't know where to make the 
necessary modifications in the Chart library.

Cheers,

  - Sacha