#3627 2d Maps: Nearly true volume of intersection weighing + riOilVolume result

* Takes the volume summation error from around 30% -> 1.5-2.0% for Norne and TEST10K_FLT_LGT_NNC for sample
spacing approximately the same as grid characteristic size.
* The latter part is for #3630 and adds a special case for calculating 2d Maps SUM of riOilVolume since it needs
to avoid using the actual riOilVolume 3d result but instead use SOIL to avoid multiplying by cell volume twice.

ApplicationCode/Application/RiaDefines.h

@@ -85,6 +85,7 @@ namespace RiaDefines
     QString combinedRiAreaNormTranResultName();
 
     QString riCellVolumeResultName();
+    QString riOilVolumeResultName();
     QString mobilePoreVolumeName();
 
     QString completionTypeResultName();