Contour plot

[bdlucas1]

• ContourPlot is implemented, supporting the Contours option and supporting two kinds of contour plot (see image).
• In the case of Automatic PlotRanges, ContourPlot and others now pass through to Graphics a PlotRange reflecting the specified {x,xmin,xmax} etc. ranges, which may be different from the ranges of x, etc. found in the emitted graphics data, so that the correct range can be plotted. This allows e.g. contour plots that may have only contours with x,y values for a subset of the specified range to be correctly plotted.
• The above tickled a bug in boxing so this was worked around.
• ContourPlot requires skimage but the existing message about missing modules wasn't showing the module name, so this was fixed.