Refactor: Change RigGridBase::cellCornerVertices to return std::array

Refactored RigGridBase::cellCornerVertices to return std::array<cvf::Vec3d, 8> by value instead of taking a reference to std::array as an output parameter. This change improves code readability and leverages modern C++ return value optimizations.

Updated all call sites across the codebase to reflect the new function signature.

ApplicationLibCode/ProjectDataModel/GeoMech/RimGeoMechCase.cpp

@@ -514,8 +514,7 @@ cvf::BoundingBox RimGeoMechCase::reservoirBoundingBox()
 
                 if ( validPorValue )
                 {
-                    std::array<cvf::Vec3d, 8> hexCorners;
-                    femPartGrid->cellCornerVertices( i, hexCorners );
+                    std::array<cvf::Vec3d, 8> hexCorners = femPartGrid->cellCornerVertices( i );
                     for ( size_t c = 0; c < 8; ++c )
                     {
                         boundingBox.add( hexCorners[c] );