Neighbor calculation and invisible face removal

As a step on the road to range filtering of Geomech cases, the removal
of internal faces is implemented, and fairly optimized.
We might consider paralellization, but reading the elements is the real
bottleneck.
Memory usage should be looked at.

ApplicationCode/GeoMech/GeoMechDataModel/RigGeoMechCaseData.cpp

@@ -87,6 +87,13 @@ bool RigGeoMechCaseData::openAndReadFemParts()
                 m_femPartResults[pIdx] = new RigFemPartResults;
                 m_femPartResults[pIdx]->initResultStages(stepNames);
             }
+
+            // Calculate derived Fem data
+            for (int pIdx = 0; pIdx < m_femParts->partCount(); ++pIdx)
+            {
+                m_femParts->part(pIdx)->assertNodeToElmIndicesIsCalculated();
+                m_femParts->part(pIdx)->assertElmNeighborsIsCalculated();
+            }
             return true;
         }
     }