Split Grid Distribution Out to Helper Functions

Mostly to reduce the number of nested scopes in doLoadBalance_() and
to make the data flow a little easier to track for human readers.
While here, also use down-casts to pointer instead of references.
Doing so replaces try/catch blocks with simpler if/else blocks.

Reimplement face transmissibility calculation/extraction loop in
terms of helper functions elements() and intersections().

ebos/eclcpgridvanguard.hh

@@ -180,7 +180,7 @@ protected:
         globalTrans_->update(false);
     }
 
-    double getTransmissibility(unsigned I, unsigned J) override
+    double getTransmissibility(unsigned I, unsigned J) const override
     {
        return globalTrans_->transmissibility(I,J);
     }