improve the ECL well model

the biggest change is that it is now based on a new approach: the well
model now always calculates the bottom hole pressure for the full well
when asked for a source term. This change makes it possible to
implement cross flow within wells properly and should also make the
well model physically correct.

Also, the well model now uses the connection transmissibility factor
which makes it possible to use this quantity if it is specified by the
deck...

examples/problems/eclproblem.hh

@@ -225,6 +225,8 @@ public:
         readMaterialParameters_();
         readInitialCondition_();
 
+        // initialize the wells. Note that this needs to be done after initializing the
+        // intrinsic permeabilities because the well model uses them...
         wellManager_.init(simulator.gridManager().eclipseState());
 
         // Start the first episode. For this, ask the Eclipse schedule.