Modernize memory management: Convert m_mainGrid to std::shared_ptr

Replace cvf::ref<RigMainGrid> with std::shared_ptr<RigMainGrid> in
RigEclipseCaseData and related classes to use standard C++ smart pointers.

Changes:
- RigEclipseCaseData: Convert m_mainGrid member to shared_ptr, add
  mainGridShared() accessor methods
- RimIdenticalGridCaseGroup: Update grid sharing logic to use shared_ptr
- RigGridManager: Store and return shared_ptr for grid deduplication
- RimEclipseStatisticsCase: Update setMainGrid() signature
- RimEclipseResultCase: Use mainGridShared() for grid copying
- RimEclipseCaseCollection: Update grid deduplication to work with shared_ptr
- RifReaderOpmCommonActive: Wrap RigActiveCellGrid in shared_ptr
- Test updates: Convert tests to use std::shared_ptr and .use_count()

Grid sharing and deduplication functionality preserved. RigMainGrid retains
cvf::Object inheritance for framework compatibility, but std::shared_ptr
handles all reference counting.

All grid-related tests pass (26/26).

ApplicationLibCode/FileInterface/RifReaderOpmCommonActive.cpp

@@ -56,7 +56,7 @@ RifReaderOpmCommonActive::~RifReaderOpmCommonActive()
 bool RifReaderOpmCommonActive::importGrid( RigMainGrid* /* mainGrid*/, RigEclipseCaseData* eclipseCaseData )
 {
     RigActiveCellGrid* activeGrid = new RigActiveCellGrid();
-    eclipseCaseData->setMainGrid( activeGrid );
+    eclipseCaseData->setMainGrid( std::shared_ptr<RigMainGrid>( activeGrid ) );
 
     caf::ProgressInfo progInfo( 4, "Importing Eclipse Grid" );