Improved Sumo summary data fetching (#14551)

Sumo data is loaded so the application stays responsive while it waits.

Run Sumo transfers on a dedicated thread
Split RiaSumoConnector into transport and data delegates
Load Sumo summary vectors concurrently and without waiting
Show that Sumo data is being loaded

ApplicationLibCode/ProjectDataModel/RimMultiPlot.h

@@ -142,6 +142,11 @@ protected:
 
     virtual void onPlotAdditionOrRemoval();
 
+    // Called before the plots are loaded, giving a window the chance to load what all of its plots need at
+    // once instead of leaving each plot to request its own. Data is still loaded on demand, so this is an
+    // optimization only: not overriding it changes nothing but speed.
+    virtual void prefetchPlotData() {}
+
     bool isMouseCursorInsidePlot();
 
 private: