Fix of GH#43 Assertion Fail for localGridIndex

The grid number associated with an ert grid was an ID and not an index.
In addition: Fixed unrobustness handling wells with no data.
Now the /Troll/MSW_LGR/2BRANCHES-CCEWELLPATH-NEW-SCH-TUNED-AR3.EGRID file loads( Have tested only two timesteps ).
p4#: 22321

ApplicationCode/ReservoirDataModel/RigMainGrid.h

@@ -44,7 +44,8 @@ public:
     size_t                                  gridCount() const           { return m_localGrids.size() + 1; }
     RigGridBase*                            gridByIndex(size_t localGridIndex);
     const RigGridBase*                      gridByIndex(size_t localGridIndex) const;
-    
+    RigGridBase*                            gridById(int localGridId);
+   
     void                                    computeCachedData();
 
     // Overrides
@@ -62,6 +63,7 @@ private:
     std::vector<cvf::Vec3d>                 m_nodes;        ///< Global vertex table
     std::vector<RigCell>                    m_cells;        ///< Global array of all cells in the reservoir (including the ones in LGR's)
     cvf::Collection<RigLocalGrid>           m_localGrids;   ///< List of all the LGR's in this reservoir
+    std::vector<size_t>                     m_gridIdToIndexMapping; ///< Mapping from LGR Id to index.
 
     cvf::Vec3d                              m_displayModelOffset;