use finishInit() properly for all problems

this means that all code which could potentially throw an exception is
moved to this method(). (In particular FluidSystem::init() proved
troublesome in the past.) Besides avoiding segmentation the faults
which stem from exceptions thrown in constructors, this also has the
advantage that simulations which spend a noticable amount of time to
initialize stop at the "correct" place, i.e. after the "Finish init of
the problem" message was printed by the simulator...

tests/problems/eclproblem.hh

@@ -216,8 +216,11 @@ public:
     EclProblem(Simulator &simulator)
         : ParentType(simulator)
         , wellManager_(simulator)
-    {}
+    { }
 
+    /*!
+     * \copydoc FvBaseProblem::finishInit
+     */
     void finishInit()
     {
         ParentType::finishInit();