Add avtPlot Method that allows plots to set Pick flags that control how pick operates and/or how pick output is displayed.  This replaces 'if (PlotTypeName == xxx) tests in ViewerQueryManager for things like glyph picking.  This is accomplished by a MapNode that is filled in with default values by avtPlot. Inividual settings can be overriden as necessary by the plots.

Changed PickAtts 'displayXXX' settings to be 'showXXX' settings for naming consitency.

Modified ScatterPlot so that it creates OriginalNodes array when needed for Picking. Scatter is currently the only plot to modify pick 'showXXX' flags.  This resolves #922

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@17670 18c085ea-50e0-402c-830e-de6fd14e8384